Medical Pharmacology: Chapter: 26 — Renal Pharmacology -- Module: Renal-04

Chapter: 26 — Renal Pharmacology — Module: Renal-04 — Renal Transplant Immunosuppression
Tier: T3 (Clinical Vignette)

1. A 38-year-old male renal transplant recipient presents seven weeks post-transplant with creatinine rising from 1.5 to 2.4 mg/dL over four days. He has no fever, graft tenderness, or decreased urine output. Tacrolimus trough is 14.2 ng/mL against a target of 8–12 ng/mL. Donor-specific antibody (DSA) testing is negative. Allograft biopsy shows tubular cell vacuolization and afferent arteriolar hyalinosis without lymphocytic tubulitis or interstitial inflammation. Which of the following is the most appropriate next step in management?

A) Initiate pulse methylprednisolone 500 mg intravenously daily for three days, as the biopsy finding of afferent arteriolar hyalinosis indicates early vascular T-cell mediated rejection (TCMR) that requires immediate corticosteroid rescue before irreversible endothelial injury occurs.
B) Add antithymocyte globulin (ATG) at 1.5 mg/kg/day for seven days to address subclinical T-cell-mediated rejection; the absence of DSA and tubulitis indicates this is a T-cell predominant process that has not yet produced sufficient lymphocytic infiltration to be detected on biopsy but is driving the creatinine rise.
C) Reduce the tacrolimus dose and recheck trough level in 48–72 hours; the supratherapeutic trough combined with biopsy findings of tubular vacuolization and afferent arteriolar hyalinosis without lymphocytic infiltration is the pathological signature of acute calcineurin inhibitor (CNI) nephrotoxicity — caused by dose-related afferent arteriolar vasoconstriction from excess thromboxane A2 and endothelin — which is reversible with dose reduction and does not require augmented immunosuppression.
D) Initiate plasmapheresis, intravenous immunoglobulin (IVIG), and rituximab, as the biopsy pattern of arteriolar injury in a DSA-negative patient represents subclinical antibody-mediated rejection (AMR) driven by non-HLA antibodies that standard DSA testing does not detect.
E) Hold tacrolimus entirely for five days and reintroduce at 50% of the original dose; complete drug holiday is necessary to allow arteriolar hyalinosis to reverse before re-exposure because tacrolimus maintains its arteriolar vasoconstrictive effect through receptor-level changes that persist after plasma levels normalize.

ANSWER: C

Rationale:

This vignette presents the classic triad of acute calcineurin inhibitor (CNI) nephrotoxicity: a supratherapeutic tacrolimus trough (14.2 ng/mL against a target of 8–12 ng/mL), rising creatinine, and a biopsy showing tubular cell vacuolization and afferent arteriolar hyalinosis without lymphocytic tubulitis or interstitial inflammation. Acute CNI nephrotoxicity results from dose-related afferent arteriolar vasoconstriction driven by excess production of thromboxane A2 (TXA2) and endothelin, reducing renal blood flow and GFR in a reversible, concentration-dependent manner. The absence of lymphocytic tubulitis and interstitial inflammation — the hallmarks of T-cell mediated rejection (TCMR) — and the negative DSA testing exclude rejection as the cause of graft dysfunction in this case. The correct intervention is tacrolimus dose reduction with close trough level monitoring; creatinine should begin improving within days of achieving therapeutic levels. Adding immunosuppression (pulse steroids, ATG, or AMR-directed therapy) to a patient with supratherapeutic CNI levels would be not only ineffective but potentially harmful by adding toxicity without addressing the cause. Option C is correct.

Option A: Option A is incorrect because afferent arteriolar hyalinosis in this context is a CNI toxicity lesion, not a TCMR vascular lesion; TCMR vascular involvement (endotheliitis, Banff v lesion) is characterized by lymphocytic intimal arteritis, not hyalinosis — and the absence of lymphocytic infiltration makes TCMR untenable.
Option B: Option B is incorrect because there is no evidence of subclinical T-cell mediated rejection; supratherapeutic CNI levels with the described biopsy pattern is a textbook presentation of CNI toxicity, not an occult T-cell process.
Option D: Option D is incorrect because the biopsy does not show the microvascular injury, peritubular capillaritis, or C4d deposition that characterize AMR; arteriolar hyalinosis is a CNI toxicity lesion, and AMR treatment is inappropriate here.
Option E: Option E is incorrect because complete drug holiday is not the management of acute CNI toxicity; dose reduction to target trough is appropriate, and arteriolar hyalinosis at this early stage does not require drug discontinuation to reverse.

2. A 52-year-old female renal transplant recipient on tacrolimus, mycophenolate mofetil (MMF), and prednisone presents with fever, diarrhea, and dysuria four months post-transplant. Her tacrolimus trough before this admission was stable at 7.2 ng/mL. She is started on ciprofloxacin for a urinary tract infection. Three days into the antibiotic course, her tacrolimus trough is 3.8 ng/mL and the transplant team notes her MMF efficacy may also be transiently reduced. Which of the following best explains both the unexpected tacrolimus trough drop and the concurrent MMF pharmacokinetic concern?

A) Severe diarrhea impairs tacrolimus gastrointestinal absorption by reducing intestinal transit time and disrupting the absorptive mucosa, causing a net decrease in tacrolimus bioavailability that outweighs any modest CYP3A4 inhibitory effect of ciprofloxacin on tacrolimus metabolism; simultaneously, ciprofloxacin eliminates intestinal flora required for bacterial deconjugation of MPA glucuronide (MPAG) back to free mycophenolic acid (MPA) during enterohepatic recirculation, reducing total MPA area under the curve and transiently lowering the antiproliferative immunosuppressive contribution of MMF.
B) Ciprofloxacin is a potent CYP3A4 inducer that dramatically increases tacrolimus first-pass metabolism, reducing the tacrolimus trough to subtherapeutic levels within 72 hours of initiation; ciprofloxacin simultaneously induces intestinal esterases to accelerate MMF hydrolysis to MPA, increasing MPA concentrations to potentially toxic levels that require MMF dose reduction during the antibiotic course.
C) The tacrolimus trough drop results from ciprofloxacin-mediated displacement of tacrolimus from FKBP12 binding sites in lymphocytes, redistributing tacrolimus from intracellular stores to plasma where it is rapidly cleared; the MMF concern arises from ciprofloxacin inhibiting P-glycoprotein efflux, trapping MPA glucuronide in enterocytes and preventing its biliary excretion for enterohepatic recirculation.
D) Ciprofloxacin chelates tacrolimus in the gastrointestinal tract before absorption, reducing tacrolimus bioavailability by approximately 50% within 24 hours; the MMF concern arises because ciprofloxacin competes with mycophenolic acid (MPA) for renal tubular secretion in the transplanted kidney, reducing MPA clearance and raising plasma MPA levels to potentially toxic concentrations requiring dose reduction.
E) The tacrolimus trough drop reflects fever-induced CYP3A4 upregulation in the liver; high fever activates hepatic nuclear factor kappa B (NF-κB), which transcriptionally induces CYP3A4 expression and accelerates tacrolimus metabolism; the MMF concern arises because fever activates intestinal esterases that accelerate conversion of MPA to its inactive glucuronide form, reducing active MPA plasma levels.

ANSWER: A

Rationale:

This vignette presents two simultaneous pharmacokinetic disruptions affecting two immunosuppressive agents in the same patient, requiring integration of tacrolimus absorption physiology and MMF enterohepatic recirculation pharmacokinetics. The tacrolimus trough drop from 7.2 to 3.8 ng/mL in the context of severe diarrhea is best explained by impaired gastrointestinal absorption: tacrolimus absorption from the intestinal lumen is highly sensitive to gastrointestinal motility, mucosal integrity, and transit time — all of which are disrupted by severe diarrhea. Reduced transit time limits contact between tacrolimus and absorptive enterocytes, while inflamed mucosa reduces absorptive surface area. Although ciprofloxacin has mild CYP3A4 inhibitory activity that might modestly raise tacrolimus levels, this effect is outweighed by the absorption impairment from the diarrheal illness itself. The concurrent MMF concern is mechanistically independent: ciprofloxacin as a broad-spectrum antibiotic depletes the intestinal flora responsible for deconjugating MPAG back to free MPA during enterohepatic recirculation. This disrupts the secondary MPA plasma peak that contributes substantially to total MPA exposure, reducing the antiproliferative immunosuppressive contribution of MMF during the antibiotic course. Both pharmacokinetic effects are real and clinically significant. Option A is correct.

Option B: Option B is incorrect because ciprofloxacin is not a CYP3A4 inducer — it has mild CYP3A4 inhibitory activity — and does not induce intestinal esterases; the described effects are the reverse of the actual pharmacology.
Option C: Option C is incorrect because ciprofloxacin does not displace tacrolimus from FKBP12 intracellular binding sites; the tacrolimus trough reflects absorption impairment, not intracellular redistribution, and ciprofloxacin does not inhibit P-gp to trap MPAG in enterocytes.
Option D: Option D is incorrect because ciprofloxacin does not chelate tacrolimus in the GI tract — this describes the antacid interaction with some drugs, not an antibiotic-tacrolimus interaction — and MPA is not meaningfully cleared by renal tubular secretion.
Option E: Option E is incorrect because fever does not meaningfully induce hepatic CYP3A4 through NF-κB activation in a clinically significant manner, and intestinal esterases are not activated by fever to increase MPA glucuronidation.

3. A 61-year-old male renal transplant recipient maintained on azathioprine, tacrolimus, and prednisone presents three years post-transplant with painful swelling of the first metatarsophalangeal joint and new subcutaneous tophi over the olecranon processes. Serum uric acid is 11.4 mg/dL. The rheumatology consultant recommends initiating allopurinol for chronic urate-lowering therapy. The transplant pharmacist urgently contacts the team before the order is placed. Which of the following correctly identifies the pharmacological hazard and the appropriate management strategy?

A) Allopurinol is safe to co-administer with azathioprine because xanthine oxidase is not involved in azathioprine metabolism at standard transplant doses; the urgent pharmacist concern relates instead to allopurinol's potent inhibition of CYP3A4, which will raise tacrolimus trough levels to nephrotoxic concentrations requiring a 50% tacrolimus dose reduction before allopurinol is initiated.
B) Allopurinol must be initiated at 25% of the standard urate-lowering dose and titrated upward over six months; at this reduced starting dose, xanthine oxidase inhibition is insufficient to cause meaningful azathioprine accumulation, and the slow titration allows TPMT to compensate for the partial xanthine oxidase blockade through increased S-methylation of 6-mercaptopurine.
C) Allopurinol can be safely co-administered with azathioprine if the azathioprine dose is reduced by 25% and complete blood count monitoring is performed weekly for the first month; the interaction causes modest thioguanine nucleotide accumulation that is manageable at the reduced azathioprine dose without requiring drug substitution.
D) Allopurinol is contraindicated with azathioprine because allopurinol inhibits TPMT directly at its active site, eliminating azathioprine inactivation and producing thioguanine nucleotide accumulation equivalent to homozygous TPMT deficiency; febuxostat is the preferred urate-lowering agent because it does not inhibit TPMT and can be used safely with azathioprine without dose adjustment.
E) Allopurinol inhibits xanthine oxidase — one of the principal enzymes responsible for catabolizing azathioprine's active thiopurine metabolites — and co-administration with azathioprine causes massive thioguanine nucleotide accumulation producing life-threatening myelosuppression even at standard azathioprine doses; the correct strategy is to substitute mycophenolate mofetil (MMF) for azathioprine before initiating allopurinol, as MMF inhibits IMPDH in the de novo purine synthesis pathway and does not depend on xanthine oxidase for inactivation.

ANSWER: E

Rationale:

This vignette presents a high-stakes drug interaction scenario where failure to recognize the pharmacological hazard could be fatal. Azathioprine is converted to 6-mercaptopurine (6-MP) and further metabolized to active thioguanine nucleotides that are myelotoxic at elevated concentrations. Two competing pathways inactivate these metabolites: S-methylation by thiopurine methyltransferase (TPMT), and oxidative catabolism by xanthine oxidase. Allopurinol inhibits xanthine oxidase to reduce uric acid production. When allopurinol blocks xanthine oxidase in a patient on azathioprine, the catabolic pathway for thiopurine metabolites is severely compromised, causing massive accumulation of thioguanine nucleotides in bone marrow progenitor cells. The clinical consequence is life-threatening pancytopenia, agranulocytosis, and fatal infections — one of the most dangerous drug interactions in transplant medicine. Even a standard 75% azathioprine dose reduction (the minimum recommended if the combination cannot be avoided) carries substantial myelosuppression risk. The correct and preferred management is to substitute azathioprine with mycophenolate mofetil (MMF) before initiating allopurinol; MMF inhibits IMPDH in the de novo purine synthesis pathway and does not involve xanthine oxidase in its metabolism or inactivation, making it pharmacologically safe to combine with allopurinol. Option E is correct.

Option A: Option A is incorrect because allopurinol does not inhibit CYP3A4 — its inhibitory activity is highly selective for xanthine oxidase — and the interaction with azathioprine is through xanthine oxidase blockade causing thioguanine nucleotide accumulation, not through CYP3A4-mediated tacrolimus elevation.
Option B: Option B is incorrect because xanthine oxidase inhibition by allopurinol at any dose causes azathioprine metabolite accumulation; there is no safe starting dose strategy that makes the combination acceptable, and TPMT cannot compensate for xanthine oxidase blockade simply by dose reduction.
Option C: Option C is incorrect because a 25% azathioprine dose reduction is grossly insufficient when xanthine oxidase is fully blocked — the recommended minimum reduction if the combination cannot be avoided is 75%, and even this carries significant risk; drug substitution is the correct approach.
Option D: Option D is incorrect because allopurinol does not inhibit TPMT directly at its active site; the mechanism is xanthine oxidase inhibition; febuxostat, while a xanthine oxidase inhibitor through a different mechanism than allopurinol, shares the same interaction risk with azathioprine and is not safe to use with azathioprine without substitution.

4. A 45-year-old female renal transplant recipient presents ten months post-transplant with a three-week history of rising creatinine from 1.3 to 2.1 mg/dL. She had a previous failed transplant eight years ago. Donor-specific antibody (DSA) testing shows a strongly positive anti-HLA class II antibody at high mean fluorescence intensity. Allograft biopsy reveals peritubular capillary C4d deposition, peritubular capillaritis, and glomerulitis without significant tubulitis. A first-year fellow suggests antithymocyte globulin (ATG) as treatment because it provides the most potent immunosuppression available. Why is ATG not appropriate here, and what treatment regimen should be initiated?

A) ATG is inappropriate because it causes complement activation that synergizes with the existing C4d complement deposition in the graft, worsening microvascular injury; the appropriate treatment is basiliximab re-induction at 20 mg intravenously on days 0 and 4 to block IL-2-driven expansion of the alloreactive T cells providing B-cell help for DSA production.
B) ATG is inappropriate because the patient's prior sensitization from her first transplant means she has pre-formed antibodies against rabbit IgG that will neutralize ATG before it can deplete T cells; the appropriate treatment is high-dose intravenous immunoglobulin (IVIG) alone at 2 g/kg as a single infusion to provide broad antibody neutralization including anti-idiotype coverage against the circulating DSAs.
C) ATG is inappropriate because the biopsy shows no tubulitis or interstitial inflammation, indicating this rejection is not T-cell driven; the appropriate treatment is pulse methylprednisolone 500 mg daily for three days, as corticosteroids suppress the complement cascade responsible for C4d deposition through NF-κB-mediated inhibition of complement gene transcription.
D) ATG is inappropriate because antithymocyte globulin depletes T cells but cannot eliminate the circulating donor-specific antibodies (DSAs) that are the primary drivers of microvascular injury in antibody-mediated rejection (AMR); the appropriate treatment is plasmapheresis to physically remove circulating DSAs, intravenous immunoglobulin (IVIG) after each session to provide immunomodulation and replacement immunoglobulins, and rituximab to deplete CD20-positive B cells and reduce de novo DSA production.
E) ATG is inappropriate because its polyclonal mechanism would deplete the regulatory T cells (Tregs) that are actively suppressing the alloantibody response; Treg depletion would paradoxically amplify DSA production and worsen AMR; the appropriate treatment is low-dose interleukin-2 (IL-2) infusion to selectively expand the surviving Treg population and restore immune tolerance to donor antigens.

ANSWER: D

Rationale:

This vignette presents antibody-mediated rejection (AMR) with its full diagnostic triad: positive donor-specific antibody (DSA class II, high mean fluorescence intensity), peritubular capillary C4d deposition (reflecting complement activation by DSA bound to graft endothelium), and microvascular injury on biopsy (peritubular capillaritis, glomerulitis). The fellow's reasoning — that ATG provides the most potent immunosuppression — misses the fundamental mechanistic distinction between AMR and T-cell mediated rejection (TCMR). ATG achieves its immunosuppressive effect by lysing circulating T cells through complement-mediated and cell-mediated cytotoxicity directed against T-cell surface antigens. While effective against TCMR, ATG has no mechanism to remove circulating DSAs from plasma, inhibit plasma cells already committed to DSA secretion, or reverse complement-mediated microvascular endothelial injury already underway. The treatment of AMR requires addressing the antibody effectors directly: plasmapheresis physically removes circulating DSAs from plasma (typically five to seven sessions); IVIG administered after each session replaces removed immunoglobulins and exerts immunomodulatory effects including reducing rebound antibody production; rituximab (anti-CD20, 375 mg/m²) depletes mature B cells and memory B cells to suppress de novo DSA production. Option D is correct.

Option A: Option A is incorrect because ATG's complement-activating mechanism does not synergistically worsen C4d deposition in a clinically meaningful way, and basiliximab re-induction is not an established AMR treatment; it targets IL-2-driven T-cell proliferation, not the antibody-mediated effector mechanism.
Option B: Option B is incorrect because pre-formed anti-rabbit IgG antibodies do not reliably neutralize ATG to prevent T-cell depletion, and IVIG alone at any dose is not the established primary AMR treatment — it is used as a component of the plasmapheresis-IVIG-rituximab regimen.
Option C: Option C is incorrect because corticosteroids suppress T-cell and macrophage inflammatory pathways through NF-κB inhibition but do not suppress the complement cascade in a manner that addresses AMR; pulse steroids are first-line treatment for TCMR, not AMR.
Option E: Option E is incorrect because low-dose IL-2 infusion to expand Tregs is an investigational approach not established as standard AMR therapy, and the concern about Treg depletion by ATG, while conceptually valid, is not the primary clinical reason ATG is inappropriate for AMR.

5. A 29-year-old female renal transplant recipient on mycophenolate mofetil (MMF) 1000 mg twice daily, tacrolimus, and prednisone calls the transplant clinic after a home pregnancy test returns positive. She is approximately six to seven weeks pregnant by last menstrual period. She was also started on trimethoprim-sulfamethoxazole (TMP-SMX) for Pneumocystis prophylaxis two weeks ago. Which of the following describes the two most urgent MMF-specific actions required at this clinic call?

A) MMF should be continued through the first trimester because the teratogenic risk applies primarily to the second and third trimesters when fetal organogenesis is complete and the fetus is larger; TMP-SMX should be discontinued immediately because it is a folate antagonist that worsens MMF-induced purine synthesis inhibition in fetal tissues, creating additive risk of neural tube defects.
B) MMF must be discontinued immediately and substituted with azathioprine, as MMF causes characteristic fetal malformations — including external ear abnormalities, cleft lip and palate, and cardiac defects — through de novo purine synthesis inhibition during organogenesis, and at six to seven weeks the patient is in the most critical window of embryonic organ formation; additionally, TMP-SMX as a broad-spectrum antimicrobial may disrupt the intestinal flora driving MMF enterohepatic recirculation and further reduce MPA exposure during the transition, which, while less immediately urgent than the teratogenicity concern, should be accounted for in managing overall immunosuppressive efficacy.
C) MMF should be reduced to the lowest effective dose (360 mg twice daily as enteric-coated mycophenolate sodium) rather than discontinued, as the FDA REMS program allows dose reduction as an alternative to discontinuation in confirmed pregnancies; TMP-SMX should be continued because Pneumocystis prophylaxis is more important than the modest MMF dose reduction's impact on fetal purine synthesis.
D) The FDA REMS program for MMF requires discontinuation only if the patient intends to continue the pregnancy; since the decision to continue or terminate the pregnancy has not yet been made, MMF should be continued at the current dose until the decision is finalized; TMP-SMX is safe in pregnancy and no changes to that medication are required at this time.
E) MMF should be held for 48 hours and restarted only after TMP-SMX is discontinued, as the combination of IMPDH inhibition by MPA and dihydrofolate reductase inhibition by trimethoprim produces additive antifolate toxicity in the mother that is the primary concern; fetal teratogenicity from MMF occurs only after 12 weeks of exposure when placental drug transport is established.

ANSWER: B

Rationale:

This vignette requires applying MMF reproductive safety pharmacology to an urgent real-world clinical scenario. At six to seven weeks gestation, the embryo is in active organogenesis — the critical window during which organ primordia including the heart, facial structures, and external ears are forming. Mycophenolic acid (MPA) inhibits de novo purine synthesis through IMPDH inhibition in rapidly dividing fetal cells, causing characteristic embryopathy: external ear and facial abnormalities, cleft lip and palate, and cardiac defects. This teratogenic risk is the basis for the FDA Risk Evaluation and Mitigation Strategy (REMS) program and requires immediate MMF discontinuation the moment pregnancy is confirmed, regardless of gestational age or decision timing. Azathioprine, while not risk-free in pregnancy, has a substantially longer clinical record in pregnant transplant recipients and is the standard antiproliferative substitution. The TMP-SMX interaction is a secondary but real pharmacokinetic consideration: as a broad-spectrum antimicrobial, TMP-SMX disrupts intestinal flora responsible for bacterial deconjugation of MPAG during enterohepatic recirculation, reducing total MPA exposure during the transition period — relevant to managing overall immunosuppressive coverage. Option B correctly identifies both the primary teratogenicity imperative and the secondary pharmacokinetic consideration.

Option A: Option A is incorrect because the teratogenic risk from MMF is greatest during the first trimester organogenesis period — precisely when this patient is at greatest risk — not the second and third trimesters; the reasoning is exactly backwards.
Option C: Option C is incorrect because no dose of MMF is acceptable in a confirmed pregnancy; there is no REMS provision allowing dose reduction as a pregnancy management strategy.
Option D: Option D is incorrect because MMF must be discontinued immediately upon confirmation of pregnancy regardless of the decision about whether to continue the pregnancy; waiting for that decision while continuing MMF exposes the embryo to ongoing teratogen during the critical organogenesis period.
Option E: Option E is incorrect because MMF teratogenicity does not begin at 12 weeks; it operates throughout organogenesis beginning from conception, and the 48-hour hold strategy does not address the ongoing teratogenic risk.

6. A 48-year-old male renal transplant recipient is converted from tacrolimus to sirolimus at post-operative week six due to early biopsy evidence of CNI nephrotoxicity. Ten days after conversion, he presents with separation of his lower transplant incision wound — a 4 cm segment has dehisced, and the urological anastomosis site shows a small perinephric fluid collection on imaging. The surgical team asks the transplant physician what caused this complication and whether sirolimus should be continued. Which of the following correctly identifies the mechanism of this complication and the appropriate management?

A) The wound dehiscence is caused by sirolimus-mediated inhibition of platelet aggregation through mTORC1-dependent thromboxane A2 synthesis in platelets; antiplatelet activity at the wound site prevents fibrin clot stabilization required for early wound tensile strength, and sirolimus should be continued with antiplatelet reversal agents.
B) The wound dehiscence is caused by sirolimus-mediated suppression of NF-κB in wound macrophages, impairing the inflammatory phase of wound healing; since macrophage-mediated debridement is impaired, the wound edges cannot be cleared of devitalized tissue and healing stalls; sirolimus should be replaced with tacrolimus and the wound managed surgically.
C) The wound dehiscence is caused by sirolimus-mediated complement inhibition at the wound site; terminal complement (C5b-9 membrane attack complex) is required for angiogenesis during wound healing, and mTORC1 inhibition suppresses complement synthesis in endothelial cells; sirolimus should be discontinued and everolimus substituted, as its shorter half-life allows faster complement recovery.
D) The wound dehiscence is caused by sirolimus inhibiting mTOR complex 1 (mTORC1), which is required for the proliferation and migration of fibroblasts and endothelial cells that execute wound repair and anastomotic healing; the conversion at six weeks was premature — mTOR inhibitors should be avoided for at least four to six weeks post-transplant until surgical healing is confirmed — and sirolimus should be discontinued with resumption of an alternative immunosuppressive regimen while the wound is managed surgically.
E) The wound dehiscence is caused by sirolimus-mediated inhibition of keratinocyte stem cell division through mTORC1-dependent epidermal growth factor receptor (EGFR) signaling at the skin surface; since sirolimus does not affect deeper fascial or anastomotic healing, the perinephric fluid collection is unrelated to the drug and represents a post-operative lymphocele that requires percutaneous drainage independently of the immunosuppression change.

ANSWER: D

Rationale:

This vignette presents the prototypical complication of premature post-transplant mTOR inhibitor initiation: surgical wound and anastomotic healing failure. mTOR complex 1 (mTORC1) signaling is required for the proliferation and migration of fibroblasts — which synthesize the collagen scaffold of healing wounds — and endothelial cells — which form the new capillaries (angiogenesis) essential for tissue oxygenation and repair. mTOR inhibition by sirolimus impairs both of these critical cellular processes throughout the healing phase, increasing the risk of wound dehiscence, anastomotic breakdown, urine leaks, and lymphocele formation around the transplanted kidney. The conversion at post-operative week six was at the outer edge of the recommended avoidance window (four to six weeks) and was premature given that wound healing was not fully confirmed. The appropriate management is sirolimus discontinuation, surgical management of the wound dehiscence and perinephric collection, and resumption of an alternative immunosuppressive regimen — typically returning to a reduced-dose CNI with nephrotoxicity monitoring. Option D correctly identifies the mTORC1-dependent fibroblast and endothelial cell proliferation mechanism and the management.

Option A: Option A is incorrect because sirolimus does not inhibit platelet aggregation through mTORC1-dependent thromboxane A2 synthesis; wound healing impairment from mTOR inhibitors is mediated through fibroblast and endothelial cell proliferation blockade, not platelet dysfunction.
Option B: Option B is incorrect because sirolimus does not suppress NF-κB in wound macrophages as its primary wound healing mechanism; NF-κB suppression is the mechanism of glucocorticoids; the mTOR inhibitor wound healing impairment is specifically about the proliferative phase of healing.
Option C: Option C is incorrect because sirolimus does not inhibit complement synthesis through mTORC1 inhibition in endothelial cells, and everolimus shares the same mTOR inhibitor class mechanism and wound healing impairment — substituting everolimus would not correct the problem.
Option E: Option E is incorrect because mTOR inhibitor wound healing impairment affects all wound components including deep fascial layers, anastomoses, and vascular structures — not just superficial keratinocytes; the perinephric fluid collection in this context is very likely a urine leak or lymphocele related to the mTOR inhibitor effect on anastomotic healing, not an independent incidental finding.

7. A 55-year-old male renal transplant recipient was converted from tacrolimus to sirolimus four months ago as part of a CNI-minimization protocol for biopsy-confirmed chronic CNI nephrotoxicity. He now presents with a six-week history of progressive exertional dyspnea and dry cough. His tacrolimus trough from before the conversion was stable; his current sirolimus trough is 7.4 ng/mL (therapeutic). Chest computed tomography (CT) shows bilateral ground-glass opacities and mild interstitial thickening bilaterally. Bronchoalveolar lavage (BAL) cultures are negative for bacteria, fungi, Pneumocystis jirovecii, and respiratory viruses. Serum creatinine remains stable. Which of the following is the most likely diagnosis, and what is the appropriate management?

A) The most likely diagnosis is sirolimus-associated non-infectious pneumonitis — a recognized class adverse effect of mTOR inhibitors caused by mTORC1 inhibition in pulmonary interstitial and immune regulatory cells — presenting with bilateral ground-glass opacities and a negative infectious workup; sirolimus must be discontinued, with most cases resolving over weeks after drug cessation, and an alternative immunosuppressive regimen should be established to maintain graft protection.
B) The most likely diagnosis is tacrolimus-induced pulmonary hypertension from the residual pharmacodynamic effect of the prior CNI on pulmonary vascular tone — tacrolimus-induced TXA2 production in pulmonary vascular smooth muscle constricts the pulmonary vasculature — and symptoms should resolve over six to eight weeks as residual tacrolimus pharmacodynamic effects wane; sirolimus should be continued at the current therapeutic trough.
C) The most likely diagnosis is subclinical antibody-mediated rejection (AMR) of the allograft presenting as pulmonary-renal syndrome; the stable creatinine reflects compensated renal function, but C4d-mediated endothelial injury has extended to the pulmonary microvasculature through circulating donor-specific antibodies; DSA testing should be performed and if positive, sirolimus should be replaced with plasmapheresis plus IVIG plus rituximab.
D) The most likely diagnosis is Pneumocystis jirovecii pneumonia (PCP) with false-negative BAL cultures due to prior TMP-SMX prophylaxis suppressing but not eliminating the organism; empirical TMP-SMX treatment should be initiated at PCP treatment doses pending repeat bronchoscopy, and sirolimus should be held temporarily because mTOR inhibition impairs the CD4 T-cell response required to clear Pneumocystis.
E) The most likely diagnosis is sirolimus-induced hyperlipidemia causing lipoid pneumonitis from alveolar VLDL (very-low-density lipoprotein) deposition; statin therapy at maximum tolerated dose should be initiated immediately, sirolimus dose should be reduced to the lower therapeutic boundary, and a lipid-lowering diet consultation should be arranged before any consideration of drug discontinuation.

ANSWER: A

Rationale:

This vignette presents the hallmark features of sirolimus-associated non-infectious pneumonitis: onset four to six weeks after mTOR inhibitor initiation or dose escalation, progressive dyspnea and dry cough, bilateral ground-glass opacities on CT, and a comprehensively negative infectious workup. Non-infectious pneumonitis is a recognized class adverse effect of mTOR inhibitors (sirolimus and everolimus) occurring in approximately 10–32% of patients, with severity ranging from asymptomatic radiographic changes to respiratory failure requiring drug discontinuation and corticosteroid treatment. The pathophysiology involves mTORC1 inhibition in pulmonary interstitial cells and immune regulatory cells impairing normal inflammatory homeostasis in the lung parenchyma. The therapeutic sirolimus trough confirms drug exposure is present and makes pneumonitis the leading diagnosis when infectious causes are excluded. Management requires sirolimus discontinuation; most cases resolve within weeks after stopping the drug, though some require a short course of corticosteroids for faster resolution. The graft must be protected with an alternative regimen during and after the transition. Option A is correct.

Option B: Option B is incorrect because tacrolimus does not cause pulmonary hypertension through TXA2-mediated pulmonary vasoconstriction, and residual tacrolimus pharmacodynamic effects do not persist four months after drug discontinuation; this mechanism is not established.
Option C: Option C is incorrect because AMR does not extend to the pulmonary microvasculature to cause pulmonary infiltrates in the described fashion, and the stable creatinine with bilateral pulmonary ground-glass opacities in an mTOR inhibitor-treated patient is characteristic of pneumonitis rather than a systemic DSA-mediated pulmonary-renal syndrome.
Option D: Option D is incorrect because the BAL negative cultures already performed are the appropriate diagnostic step; empirical PCP treatment on clinical grounds alone is not supported when the full negative workup including BAL cultures is available, and the clinical context strongly favors mTOR inhibitor pneumonitis over PCP.
Option E: Option E is incorrect because sirolimus does cause hyperlipidemia, but hyperlipidemia does not cause lipoid pneumonitis through VLDL alveolar deposition — this is not a recognized mechanism of sirolimus pulmonary toxicity; the correct diagnosis is non-infectious pneumonitis requiring drug discontinuation, not dose reduction with statin addition.

8. A 44-year-old female renal transplant recipient with a stable tacrolimus trough of 6.5 ng/mL is started on carbamazepine by her neurologist for newly diagnosed trigeminal neuralgia. Three weeks later, she presents with an asymptomatic creatinine rise from 1.2 to 1.8 mg/dL. Tacrolimus trough is now 2.1 ng/mL. Which of the following correctly identifies the mechanism of this drug interaction and describes the most appropriate management strategy?

A) Carbamazepine inhibits cytochrome P450 3A4 (CYP3A4) in the intestinal wall, reducing tacrolimus first-pass metabolism and causing tacrolimus accumulation to supratherapeutic levels; however, the distribution volume of tacrolimus is so large that the plasma trough does not reflect true tissue exposure — a trough of 2.1 ng/mL may represent adequate tissue calcineurin inhibition despite appearing subtherapeutic by standard assay, and dose reduction rather than escalation should be considered.
B) Carbamazepine directly inhibits the calcineurin enzyme through a sodium channel-independent mechanism, reducing the immunosuppressive efficacy of tacrolimus despite a normal or low trough level; the tacrolimus dose should be maintained at current levels and carbamazepine discontinued immediately to restore calcineurin inhibitory activity.
C) Carbamazepine is a potent inducer of cytochrome P450 3A4 (CYP3A4) and P-glycoprotein (P-gp), dramatically increasing tacrolimus first-pass metabolism and intestinal efflux and reducing the tacrolimus trough to a critically subtherapeutic level that places the graft at acute rejection risk; the appropriate management is urgent tacrolimus dose escalation guided by daily trough monitoring, combined with consideration of substituting carbamazepine with an alternative anticonvulsant — such as gabapentin or pregabalin — that does not induce CYP3A4.
D) Carbamazepine reduces tacrolimus bioavailability by chelating tacrolimus in the alkaline pH of the small intestine; the clinical management is to separate carbamazepine and tacrolimus administration by at least six hours, after which the tacrolimus trough is expected to return to baseline without requiring dose adjustment.
E) Carbamazepine displaces tacrolimus from plasma protein binding sites, increasing the free (unbound) tacrolimus fraction while reducing the total tacrolimus trough measured by standard whole-blood assay; since immunosuppressive efficacy is determined by free drug concentration rather than total trough, no dose adjustment is required despite the apparently low trough, and the creatinine rise reflects rejection from reduced calcineurin inhibitor tissue levels that the assay underestimates.

ANSWER: C

Rationale:

This vignette presents a clinically important CYP3A4 drug interaction that mirrors the rifampin-tacrolimus interaction in mechanism. Carbamazepine is a potent inducer of cytochrome P450 3A4 (CYP3A4) and P-glycoprotein (P-gp) — two of the principal determinants of tacrolimus bioavailability and systemic clearance. By upregulating intestinal and hepatic CYP3A4, carbamazepine dramatically accelerates tacrolimus first-pass metabolism, and by upregulating intestinal P-gp, it increases efflux of tacrolimus back into the gut lumen. The combined inductive effect reduces the tacrolimus trough from a therapeutic 6.5 ng/mL to a critically subtherapeutic 2.1 ng/mL within three weeks — the typical time course for full CYP3A4 induction to become established. A trough of 2.1 ng/mL provides insufficient calcineurin inhibition to prevent T-cell activation against donor alloantigens, and the creatinine rise of 1.2 to 1.8 mg/dL suggests early rejection is underway. Management requires urgent tacrolimus dose escalation with daily trough monitoring; dose increases of two- to three-fold may be required to restore therapeutic levels in the presence of ongoing carbamazepine-induced CYP3A4 activity. Simultaneously, substituting carbamazepine with an alternative anticonvulsant that does not induce CYP3A4 — such as gabapentin or pregabalin for trigeminal neuralgia — eliminates the interaction entirely and is the preferred long-term strategy. Option C is correct.

Option A: Option A is incorrect because carbamazepine is a CYP3A4 inducer, not an inhibitor, and the low trough of 2.1 ng/mL reflects real subtherapeutic drug exposure requiring dose escalation, not a tissue distribution artifact that makes dose reduction appropriate.
Option B: Option B is incorrect because carbamazepine does not inhibit calcineurin; it acts as a sodium channel blocker for its anticonvulsant effect and as a CYP3A4/P-gp inducer for its pharmacokinetic interaction with tacrolimus — there is no direct calcineurin interaction.
Option D: Option D is incorrect because carbamazepine does not chelate tacrolimus in the small intestinal lumen; the interaction is enzymatic and transporter-mediated through CYP3A4/P-gp induction, and separating administration times does not correct an induction-based interaction.
Option E: Option E is incorrect because tacrolimus is measured in whole blood (bound to erythrocytes and plasma proteins), and carbamazepine does not meaningfully displace tacrolimus from protein binding to create an artifactually low total trough; the low trough reflects real reduction in drug exposure requiring dose escalation.

9. A 67-year-old male renal transplant recipient maintained on prednisone 5 mg daily, tacrolimus, and MMF for five years presents for elective total hip replacement. The orthopedic anesthesiologist reviews the medication list and asks the transplant team whether the patient requires perioperative stress-dose corticosteroids. The transplant physician states they are required. The anesthesiologist questions this, noting that the patient is already on a daily steroid and asks why 5 mg prednisone per day is insufficient to cover surgical stress. Which of the following best explains the physiological rationale for perioperative stress-dose corticosteroids in this patient?

A) Prednisone 5 mg daily is insufficient because surgical anesthesia agents — particularly propofol and volatile anesthetics — competitively inhibit glucocorticoid receptor binding in the hypothalamus, transiently blocking the ability of exogenous prednisone to suppress the surgical pain response; stress-dose corticosteroids overcome this receptor competition and restore adequate hypothalamic glucocorticoid signaling during the procedure.
B) Prednisone 5 mg daily is insufficient because CYP3A4-mediated prednisone metabolism is dramatically upregulated by the metabolic stress of surgery; hepatic CYP3A4 activity increases three- to five-fold in response to surgical trauma through NF-κB-mediated enzyme induction, reducing prednisone plasma levels to sub-therapeutic concentrations within hours of incision.
C) Prednisone 5 mg daily is insufficient because tacrolimus competitively blocks glucocorticoid receptor binding at therapeutic trough concentrations; co-administration of tacrolimus requires two- to three-fold higher prednisone doses to achieve the same receptor occupancy, and the 5 mg maintenance dose is pharmacodynamically neutralized by chronic tacrolimus exposure.
D) Prednisone 5 mg daily is insufficient because the anti-inflammatory dose required to prevent surgical site rejection is 60–80 mg prednisone equivalent per day; the maintenance dose of 5 mg suppresses the immune response adequately at rest but cannot prevent T-cell-mediated graft injury triggered by the pro-inflammatory cytokine release that accompanies major orthopedic surgery.
E) Chronic administration of exogenous prednisone — even at low doses — suppresses the hypothalamic-pituitary-adrenal (HPA) axis through negative feedback, causing adrenal cortical atrophy and severely impairing or eliminating the adrenal gland's ability to mount an endogenous cortisol surge; under major surgical stress, a physiologically intact adrenal gland would produce approximately 75–150 mg of hydrocortisone equivalent per day — far exceeding what 5 mg prednisone provides — and without exogenous stress-dose supplementation the patient risks adrenal crisis with refractory hypotension, hyponatremia, and cardiovascular collapse.

ANSWER: E

Rationale:

This vignette presents a classic and clinically critical scenario involving HPA axis suppression from chronic exogenous corticosteroid administration. The anesthesiologist's reasoning — that a patient already on daily prednisone does not need additional steroid coverage — misunderstands the distinction between maintenance dosing and stress-response dosing. Exogenous prednisone at 5 mg daily provides negative feedback to the hypothalamus (reducing CRH secretion) and the anterior pituitary (reducing ACTH secretion), causing progressive atrophy of the adrenal cortex's cortisol-secreting zona fasciculata over months to years. The result is an adrenal gland that cannot mount the cortisol surge that surgical stress demands. A physiologically normal HPA axis responds to major surgery by generating the equivalent of 75–150 mg hydrocortisone per day — five to thirty times the maintenance dose. Without this surge, the patient faces relative adrenal insufficiency during the perioperative period: refractory vasodilatory hypotension unresponsive to standard vasopressors, hyponatremia from relative aldosterone insufficiency, hypoglycemia, and in severe cases fatal cardiovascular collapse. Stress-dose corticosteroids (typically hydrocortisone 50–100 mg IV at induction followed by 25–50 mg every 8 hours for 24–48 hours) replace the endogenous surge that the suppressed HPA axis cannot generate. Option E correctly articulates this mechanism.

Option A: Option A is incorrect because anesthetic agents do not competitively inhibit glucocorticoid receptor binding in the hypothalamus; the requirement for stress-dose steroids is based on HPA axis suppression physiology, not pharmacological receptor competition from anesthetics.
Option B: Option B is incorrect because surgical trauma does not meaningfully induce hepatic CYP3A4 through NF-κB activation in a way that accelerates prednisone clearance; CYP3A4 inducers are specific drugs, not metabolic states from surgery.
Option C: Option C is incorrect because tacrolimus does not competitively block glucocorticoid receptors; it binds FKBP12 and inhibits calcineurin — an entirely different molecular pathway with no glucocorticoid receptor interaction.
Option D: Option D is incorrect because the requirement for stress-dose steroids is physiological — replacing the missing endogenous cortisol surge — not immunological; the concern is adrenal crisis from HPA axis suppression, not transplant rejection from inadequate anti-inflammatory dosing.

10. A 50-year-old male receiving his second renal transplant with a panel reactive antibody (PRA) of 58% and detected donor-specific antibodies is given antithymocyte globulin (ATG) induction. During the second ATG infusion on post-operative day two, the patient develops fever to 39.2°C, severe rigors, and blood pressure of 78/46 mmHg requiring IV fluid resuscitation. The infusion is held and the patient stabilizes over two hours. A nurse asks why the patient was not premedicated before this infusion and what premedication is required before all subsequent ATG doses. Which of the following correctly explains the mechanism of this reaction and the required premedication regimen?

A) This reaction represents an IgE-mediated Type I hypersensitivity reaction to the rabbit protein component of ATG; since desensitization has now occurred with the second-dose reaction, subsequent infusions can proceed without premedication, though a slower infusion rate and bedside epinephrine availability are required as precautions.
B) This reaction represents complement consumption syndrome from massive C3 depletion during ATG-mediated T-cell lysis; complement consumption reduces opsonization capacity and predisposes to bacterial infection during subsequent infusions; prophylactic antibiotics should be added to the premedication regimen along with eculizumab to protect complement reserves during the ATG course.
C) This reaction represents tacrolimus-ATG pharmacodynamic synergy — concurrent calcineurin inhibition by tacrolimus sensitizes T cells to ATG-mediated lysis, amplifying cytokine release beyond what ATG alone would produce; tacrolimus should be held for 48 hours before each subsequent ATG infusion to reduce the sensitization effect, and acetaminophen given 30 minutes before infusion.
D) This reaction represents a cytokine release syndrome caused by massive pro-inflammatory cytokine liberation — tumor necrosis factor alpha, interleukin-6, and other mediators — as the polyclonal ATG antibodies lyse large numbers of circulating T cells through complement-mediated and Fc-receptor-mediated cytotoxicity; premedication before every ATG infusion must include a corticosteroid (methylprednisolone), acetaminophen, and an antihistamine administered approximately 30–60 minutes before each dose to attenuate the cytokine release and inflammatory response.
E) This reaction represents acute serum sickness from the rapid formation of immune complexes between rabbit IgG (in ATG) and the patient's pre-formed anti-rabbit antibodies from prior ATG exposure; since serum sickness worsens with each ATG dose, the ATG course should be discontinued and basiliximab initiated as an alternative induction strategy that avoids the rabbit protein trigger.

ANSWER: D

Rationale:

This vignette presents cytokine release syndrome — the expected and anticipated adverse effect of ATG infusion — and requires recognition of both the mechanism and the preventive premedication strategy. Antithymocyte globulin (ATG) is a polyclonal antibody preparation directed against a broad array of T-cell surface antigens. When ATG binds to circulating T cells, it activates complement through the classical pathway and triggers Fc-receptor-mediated cytotoxicity, causing rapid lysis of large numbers of T cells. As T cells lyse, they release massive quantities of pro-inflammatory cytokines including tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), and interferon-gamma (IFN-γ) into the systemic circulation. This cytokine surge produces the cytokine release syndrome: fever, rigors, hypotension, tachycardia, and in severe cases, capillary leak. Because this reaction is mechanistically expected and not an allergic response, premedication must be administered before every ATG infusion — not only the first — to attenuate the cytokine-mediated inflammatory response. Standard premedication includes a corticosteroid (typically methylprednisolone 1–2 mg/kg IV), acetaminophen (650–1000 mg orally or rectally), and an antihistamine (diphenhydramine), given 30–60 minutes before infusion. Slowing the infusion rate also reduces peak cytokine release. This patient was not premedicated before the second infusion, allowing the full cytokine release syndrome to develop. Option D is correct.

Option A: Option A is incorrect because cytokine release syndrome from ATG is not an IgE-mediated Type I hypersensitivity reaction; it is a pharmacodynamic consequence of T-cell lysis; it does not represent desensitization and premedication remains required for every subsequent dose.
Option B: Option B is incorrect because complement consumption syndrome with bacterial infection risk is not the mechanism of ATG infusion reactions, and eculizumab is not a premedication for ATG infusion; the reaction is cytokine-mediated, not complement-consumption-mediated.
Option C: Option C is incorrect because tacrolimus does not sensitize T cells to ATG-mediated lysis in a way that amplifies cytokine release beyond ATG alone; holding tacrolimus before ATG infusions is not standard practice and would not prevent cytokine release syndrome.
Option E: Option E is incorrect because this is not serum sickness from anti-rabbit antibodies; serum sickness is a delayed hypersensitivity reaction occurring 7–21 days after exposure, not an acute infusion reaction during the second dose; furthermore, prior ATG exposure is not documented in this patient's history, and continuing ATG with proper premedication is the correct approach.

11. A 36-year-old female renal transplant recipient received high-dose methylprednisolone 500 mg intravenously at transplant and standard pulse steroid induction 18 months ago, and has been maintained on prednisone 5 mg daily since. She now presents with a five-month history of right hip pain that began insidiously and worsens progressively with weight-bearing and stair climbing. She denies fever, swelling, or erythema over the joint. Plain radiograph of the right hip is reported as normal by the radiologist. The transplant physician suspects a specific corticosteroid complication. Which of the following correctly identifies the suspected diagnosis, the appropriate confirmatory imaging, and the mechanism linking high-dose corticosteroid exposure to this condition?

A) The suspected diagnosis is corticosteroid-induced osteoporosis with femoral neck insufficiency fracture; plain radiograph can miss non-displaced fractures, and nuclear medicine bone scintigraphy (technetium-99m bone scan) is the most sensitive confirmatory imaging; the mechanism is osteoblast suppression combined with increased osteoclast activity reducing cortical bone density at high-stress sites.
B) The suspected diagnosis is avascular necrosis (osteonecrosis) of the femoral head — a complication of high-dose corticosteroid exposure in which impaired vascular supply to subchondral bone causes osteocyte death and eventual structural collapse; plain radiograph is insensitive for early disease and frequently normal, and magnetic resonance imaging (MRI) is the gold standard for diagnosis, detecting characteristic subchondral signal changes before radiographic collapse occurs; the mechanism involves corticosteroid-induced fat cell hypertrophy increasing intraosseous pressure, intravascular fat embolism occluding terminal bone arteries, and direct corticosteroid-mediated osteocyte apoptosis.
C) The suspected diagnosis is septic arthritis of the hip from an opportunistic pathogen; immunosuppressed transplant recipients are at risk for indolent fungal or mycobacterial joint infection that produces progressive pain without the acute features of bacterial septic arthritis; ultrasound-guided joint aspiration with culture for bacteria, fungi, and mycobacteria is the confirmatory diagnostic step.
D) The suspected diagnosis is corticosteroid-induced proximal myopathy of the hip girdle musculature; prednisone-mediated glucocorticoid receptor activation causes type IIb muscle fiber atrophy in the gluteal and hip flexor muscles, producing pain with weight-bearing that is mislocalized to the hip joint; electromyography (EMG) is the confirmatory test and will show myopathic changes without evidence of denervation.
E) The suspected diagnosis is calcineurin inhibitor-induced gout with hip joint involvement; tacrolimus-mediated reduction in renal urate excretion combined with long-term low-dose prednisone produces hyperuricemia with atypical joint deposition; joint aspiration with polarized light microscopy for monosodium urate crystals is the confirmatory test, and allopurinol initiation — after azathioprine is substituted with MMF — is the appropriate treatment.

ANSWER: B

Rationale:

This vignette presents the characteristic clinical signature of avascular necrosis (AVN, also called osteonecrosis) of the femoral head: insidious onset of progressive weight-bearing hip pain beginning months after high-dose corticosteroid exposure, in a patient without systemic signs of infection or inflammation, with a normal plain radiograph. AVN is an underrecognized but well-established complication of high-dose corticosteroid administration in transplant recipients. The mechanism is multifactorial: glucocorticoid-induced hypertrophy of fat cells within the rigid intraosseous compartment increases marrow pressure and compresses terminal vascular channels; fat emboli from corticosteroid-mobilized adipose tissue occlude the terminal arteries supplying the femoral head subchondral bone; and direct glucocorticoid receptor-mediated suppression of osteocyte survival reduces the cells responsible for maintaining bone vascularity. The result is progressive osteocyte death in the femoral head, followed by subchondral bone collapse. Plain radiographs are insensitive in early AVN and are typically normal for months after symptom onset. MRI is the gold standard for early diagnosis, detecting characteristic T1 and T2 signal abnormalities in the subchondral bone before any radiographic change occurs. Early diagnosis allows consideration of core decompression to reduce intraosseous pressure before femoral head collapse necessitates total hip replacement. Option B correctly identifies the diagnosis, imaging modality, and mechanism.

Option A: Option A is incorrect because while osteoporosis with insufficiency fracture is also a corticosteroid complication, the mechanism, clinical timeline, and appropriate imaging differ; AVN is a higher-priority diagnosis given the five-month progressive course with normal X-ray, and MRI — not bone scan — is the preferred next imaging.
Option C: Option C is incorrect because the five-month insidious progressive course without fever, effusion, or systemic signs makes indolent infectious arthritis much less likely than AVN; the temporal relationship to high-dose induction steroids is the dominant clinical feature.
Option D: Option D is incorrect because corticosteroid myopathy presents as proximal muscle weakness — difficulty rising from chairs, climbing stairs by pushing with arms — rather than focal joint pain; the pain localizes to the hip joint in this vignette, pointing to articular rather than muscular pathology.
Option E: Option E is incorrect because hip gout is exceedingly rare, and while calcineurin inhibitors do cause hyperuricemia, the five-month progressive weight-bearing pain following high-dose corticosteroid exposure in a post-transplant patient is AVN until proven otherwise.