Medical Pharmacology Question Bank

Chapter 1: General Pharmacology — Module 4: Adverse Effects and Drug Interactions
Tier: Tier 3 — Clinical Vignettes

1. A 34-year-old woman with systemic lupus erythematosus presents to the emergency department with urticaria, lip swelling, bronchospasm, and hypotension developing within ten minutes of receiving intravenous trimethoprim-sulfamethoxazole for a Pneumocystis jirovecii pneumonia prophylaxis breakthrough infection. She had previously received oral TMP-SMX without incident two years ago. Her blood pressure is 78/42 mmHg and oxygen saturation is 88% on room air. Which of the following best identifies the immunological reaction type and the most critical initial pharmacological intervention?

A) This is a Gell-Coombs Type III immune complex hypersensitivity reaction; the most critical intervention is intravenous hydrocortisone 200 mg to suppress complement activation and immune complex deposition in vessel walls
B) This is a Gell-Coombs Type IV delayed hypersensitivity reaction mediated by sensitized T lymphocytes; the most critical initial intervention is oral prednisolone 40 mg and antihistamine therapy, with observation for 48–72 hours for peak T cell-mediated inflammation
C) This is a Gell-Coombs Type II cytotoxic hypersensitivity reaction; IgG antibodies targeting TMP-SMX-coated erythrocytes are causing hemolytic anemia and the cardiovascular compromise; the most critical intervention is platelet transfusion and corticosteroids
D) This is a Gell-Coombs Type I immediate hypersensitivity reaction (anaphylaxis) mediated by IgE cross-linking on mast cells and basophils; the most critical and life-saving initial pharmacological intervention is intramuscular epinephrine (adrenaline) 0.5 mg (1:1000 solution), which reverses bronchospasm via beta-2 adrenoceptor activation, restores vasomotor tone via alpha-1 adrenoceptor activation, and suppresses further mediator release
E) This is a non-immunological (pseudoallergic) direct mast cell degranulation reaction that does not require epinephrine; antihistamines alone are sufficient for management because IgE is not involved and the reaction cannot progress to cardiovascular collapse

ANSWER: D

Rationale:

The clinical presentation — urticaria, angioedema (lip swelling), bronchospasm, and hypotension developing within ten minutes of IV drug administration — fulfills the diagnostic criteria for anaphylaxis: a severe, life-threatening generalized hypersensitivity reaction. The prior uneventful exposure to oral TMP-SMX two years earlier is consistent with the two-exposure requirement of Type I IgE-mediated sensitization: the first exposure generated TMP-SMX-specific IgE antibodies that bound to FcRI on mast cells and basophils; the current IV exposure re-introduced antigen that cross-linked adjacent IgE molecules, triggering rapid degranulation and release of histamine, tryptase, prostaglandin D2, and leukotriene C4/D4/E4. Intravenous administration accelerates antigen delivery and typically produces more severe reactions than oral routes. The most critical, life-saving, and time-sensitive initial intervention is intramuscular epinephrine 0.5 mg of 1:1000 solution into the anterolateral thigh. Epinephrine acts through multiple adrenoceptor mechanisms simultaneously: alpha-1 adrenoceptor activation on vascular smooth muscle restores vasomotor tone, raises systemic vascular resistance, and reverses the distributive shock component; beta-2 adrenoceptor activation on bronchial smooth muscle reverses bronchospasm; beta-1 adrenoceptor activation increases cardiac output; alpha-1 and beta-2 effects on mast cells and basophils suppress further mediator release. Antihistamines (H1 and H2 blockers) and corticosteroids are important adjuncts but are not the critical immediate intervention — delayed epinephrine administration in anaphylaxis is associated with preventable death. Option A is incorrect — Type III reactions (serum sickness, vasculitis) present with fever, urticaria, arthralgias, and lymphadenopathy 1–3 weeks after exposure, not within minutes. Option B is incorrect — Type IV reactions peak at 48–72 hours after exposure; prednisolone is not the first-line emergency intervention for anaphylaxis. Option C is incorrect — Type II cytotoxic reactions cause cell destruction (hemolytic anemia, thrombocytopenia) through IgG/IgM-complement mechanisms, not acute bronchospasm and cardiovascular collapse within minutes. Option E is critically incorrect and dangerous — whether the mechanism is IgE-mediated (Type I) or pseudoallergic (direct mast cell activation), the clinical management of anaphylaxis is identical and requires epinephrine; antihistamines alone are never adequate treatment for anaphylaxis with cardiovascular compromise, and the risk of fatal progression is real in either mechanism.

2. A 58-year-old man undergoes coronary artery bypass grafting and is anticoagulated postoperatively with unfractionated heparin. On postoperative day seven, his platelet count falls from 224 × 10/L (preoperative baseline) to 68 × 10/L. He remains afebrile with no signs of bleeding, but develops acute right calf pain and swelling. Doppler ultrasound confirms a right popliteal deep vein thrombosis. His 4T score for HIT is calculated at 7 (high probability). Which of the following represents the most pharmacologically sound immediate management?

A) Administer platelet transfusion to raise the platelet count above 100 × 10/L before initiating therapeutic anticoagulation, as thrombocytopenia must be corrected before anticoagulant therapy can safely prevent further thrombosis
B) Continue heparin at the current dose and add warfarin immediately to bridge to therapeutic INR, as the DVT requires anticoagulation and heparin remains the safest agent in the perioperative setting
C) Discontinue heparin immediately, do not administer platelet transfusions, initiate a non-heparin anticoagulant (argatroban or fondaparinux), send anti-PF4 antibody testing, and avoid all heparin products including low-molecular-weight heparin until HIT is excluded or the diagnosis is confirmed and resolved
D) Discontinue heparin and initiate low-molecular-weight heparin (enoxaparin) instead, as LMWH has a lower risk of HIT than unfractionated heparin and can be safely used to treat the DVT while the HIT resolves
E) Withhold all anticoagulation until platelet count recovers above 150 × 10/L to avoid worsening thrombocytopenia, then resume anticoagulation with warfarin at therapeutic INR for DVT treatment

ANSWER: C

Rationale:

This case presents classic HIT with thrombosis (HITT) — the most dangerous complication of HIT and a pharmacological emergency. The 4T score of 7 represents high pre-test probability for HIT (>80% positive predictive value), and the clinical picture is entirely consistent: platelet count falling >50% from baseline between days five and ten of heparin therapy (the typical immunological window for HIT antibody formation), with paradoxical thrombosis (DVT) despite ongoing anticoagulation. Management is dictated by the pharmacological mechanism of HIT: IgG anti-PF4-heparin antibodies activate platelets via FcRIIA, generating a profoundly prothrombotic state. Every aspect of management follows from this mechanism. Immediate heparin cessation is mandatory — continuing heparin in the presence of HIT antibodies perpetuates platelet activation and thrombosis risk; even "flushing" catheters with heparin is contraindicated. LMWH is contraindicated in HIT — anti-PF4 HIT antibodies cross-react with LMWH-PF4 complexes in approximately 90% of cases, meaning LMWH does not escape the immunological mechanism. Platelet transfusion is contraindicated — infusing platelets in HIT provides substrate fuel for the anti-PF4 IgG-FcRIIA activation mechanism, potentially worsening thrombosis. A non-heparin anticoagulant must be started immediately: argatroban (a direct thrombin inhibitor, renally excreted — preferred in hepatic impairment) or bivalirudin are the agents of choice in the perioperative setting; fondaparinux (a synthetic pentasaccharide Factor Xa inhibitor) does not cross-react with anti-PF4 antibodies and is an acceptable alternative. Warfarin must not be started alone or until the platelet count has substantially recovered (>150 × 10/L) — initiating warfarin during the acute HIT platelet nadir causes protein C depletion (protein C has a shorter half-life than Factor II), precipitating venous limb gangrene (a catastrophic complication). Anti-PF4 antibody testing (ELISA) confirms the diagnosis but management cannot wait for results when pre-test probability is high. Option A is incorrect — platelet transfusion in HIT is actively harmful, not a prerequisite for anticoagulation. Option B is incorrect — continuing heparin perpetuates the HIT mechanism; immediate warfarin initiation without bridging in active HIT risks protein C depletion and venous limb gangrene. Option D is incorrect — LMWH cross-reacts with HIT antibodies and cannot substitute for non-heparin anticoagulation. Option E is incorrect — withholding all anticoagulation in HITT leaves the patient at extreme thrombotic risk; anticoagulation with a non-heparin agent must begin immediately regardless of platelet count.

3. A 44-year-old man with schizophrenia, type 2 diabetes, and hypertension presents to clinic for medication review. His current medications include clozapine 400 mg daily, metformin 1000 mg twice daily, and amlodipine 10 mg daily. His physician considers adding a macrolide antibiotic for a community-acquired respiratory infection. Erythromycin is initially selected but then changed to azithromycin. The physician also notes that clozapine carries a well-established risk of QT prolongation. Which of the following best explains why erythromycin poses a greater combined QT and pharmacokinetic risk than azithromycin in this specific patient, and what additional electrolyte factor must be checked before prescribing either macrolide?

A) Erythromycin is a more potent direct hERG channel blocker than azithromycin, and is also a potent CYP3A4 inhibitor that would increase clozapine plasma concentrations; the combination produces additive QT prolongation through both direct hERG blockade and clozapine accumulation via CYP inhibition; azithromycin also blocks hERG but does not inhibit CYP3A4, carrying only the direct QT effect without the pharmacokinetic amplification; serum potassium and magnesium must be checked and corrected before prescribing either macrolide, as hypokalemia and hypomagnesemia are independent risk factors that sensitize hERG channels to drug blockade
B) Erythromycin causes QT prolongation exclusively through CYP3A4 inhibition of clozapine metabolism, with no direct hERG-blocking activity of its own; azithromycin is a direct hERG blocker but does not inhibit CYP enzymes; the combined risk is purely pharmacokinetic with erythromycin and purely pharmacodynamic with azithromycin
C) Azithromycin poses greater risk than erythromycin because its longer half-life (68 hours vs erythromycin's 1.5 hours) prolongs QT exposure for days after the last dose; erythromycin's short half-life limits total QT risk to the duration of active dosing only
D) The two macrolides have identical QT and CYP interaction profiles and are pharmacologically interchangeable in this patient; the drug switch provides no pharmacological advantage and the prescriber's concern is not justified
E) Clozapine does not prolong the QT interval; the QT concern in this patient arises entirely from amlodipine, which blocks cardiac L-type calcium channels and prolongs ventricular repolarization; macrolide selection should be based on antibacterial spectrum alone

ANSWER: A

Rationale:

This case requires integrating pharmacokinetic (CYP inhibition) and pharmacodynamic (additive hERG blockade) drug interaction principles in a patient with multiple QT risk factors. Erythromycin carries a dual QT risk in this patient: first, it is a well-established direct hERG channel blocker that independently prolongs the QT interval — erythromycin-induced TdP is a well-documented clinical entity that initially prompted widespread awareness of macrolide cardiac risk. Second, erythromycin is a potent CYP3A4 inhibitor (mechanism-based inhibitor via nitroso-alkane intermediate formation). Clozapine is metabolized in part by CYP3A4 (primarily by CYP1A2, but CYP3A4 contributes meaningfully); erythromycin-mediated CYP3A4 inhibition would increase clozapine plasma concentrations, amplifying clozapine's own hERG-blocking activity. The result is a pharmacokinetic-pharmacodynamic cascade: more clozapine more clozapine-mediated hERG blockade more QT prolongation, layered on top of erythromycin's own direct hERG blockade. Azithromycin is also a hERG channel blocker (carrying its own direct QT prolongation risk, as established by the NEJM Azithromycin risk publications and FDA safety communications), but crucially, azithromycin does not meaningfully inhibit CYP3A4 at therapeutic concentrations — it lacks the pharmacokinetic amplification component. Azithromycin therefore carries only the direct additive hERG blockade risk when combined with clozapine, not the compounded risk of clozapine accumulation. Before prescribing either macrolide, serum potassium and magnesium must be measured and corrected: hypokalemia reduces the electrochemical driving force for IKr and sensitizes hERG channels to drug blockade; hypomagnesemia independently promotes EAD formation and is the first-line pharmacological treatment for TdP if it occurs. This patient with diabetes (risk of hypokalemia from polyuria, diabetic ketoacidosis) requires electrolyte assessment as a baseline safety check. Option B is incorrect — erythromycin is both a direct hERG blocker and a CYP3A4 inhibitor; azithromycin is a direct hERG blocker but not a clinically relevant CYP inhibitor. Option C is incorrect — while azithromycin's long half-life does prolong its QT-prolonging exposure, this does not make it more dangerous than erythromycin's combined direct hERG blockade plus CYP-mediated pharmacokinetic amplification in a patient on a QT-prolonging substrate. Option D is incorrect — erythromycin and azithromycin have different CYP inhibition profiles that are pharmacologically and clinically distinct, particularly in patients on CYP3A4-metabolized QT-prolonging drugs. Option E is incorrect — clozapine has well-documented QT-prolonging activity through hERG blockade; amlodipine is a dihydropyridine calcium channel blocker that does not clinically prolong the QT interval.

4. A 77-year-old woman with hypertension, chronic atrial fibrillation, and osteoarthritis is prescribed naproxen 500 mg twice daily by her orthopedic surgeon for knee pain. She is also taking apixaban 5 mg twice daily for AF stroke prevention. Three weeks later she is admitted with hematemesis and melena. Endoscopy reveals a bleeding gastric ulcer. Her hemoglobin is 7.2 g/dL. Which of the following best characterizes the pharmacodynamic drug interaction responsible for this patient's gastrointestinal hemorrhage, and identifies the most appropriate analgesic alternative?

A) Naproxen inhibits CYP2C9, reducing apixaban metabolism and increasing its plasma concentration to supratherapeutic levels — this is a pharmacokinetic interaction; the appropriate alternative is ibuprofen, which does not inhibit CYP2C9
B) Naproxen is a non-selective COX inhibitor that reduces prostaglandin E2 synthesis in the gastric mucosa, impairing mucosal cytoprotection (reduced mucus and bicarbonate secretion, reduced mucosal blood flow) and facilitating ulcer formation; concurrent apixaban — a direct Factor Xa inhibitor — impairs hemostasis by reducing thrombin generation and fibrin clot formation, preventing effective hemostasis at the ulcer base; this is a pharmacodynamic interaction: naproxen damages the mucosal barrier (facilitating bleeding) while apixaban impairs the hemostatic response (preventing bleeding cessation); the most appropriate analgesic alternative is acetaminophen (paracetamol), which does not inhibit COX in the gastric mucosa at standard doses and carries no antiplatelet or mucosal toxic effect
C) Apixaban inhibits CYP3A4, reducing naproxen metabolism and increasing its plasma concentration to levels that cause direct gastric mucosal erosion independent of prostaglandin inhibition — this is a pharmacokinetic interaction; the appropriate alternative is celecoxib, which is not metabolized by CYP3A4
D) The gastrointestinal hemorrhage is a predictable Type A adverse effect of apixaban alone at standard doses in elderly patients; naproxen plays no pharmacodynamic role in the bleeding event and can be safely continued with the addition of a proton pump inhibitor
E) Naproxen and apixaban cause gastrointestinal hemorrhage through identical mechanisms — both inhibit thromboxane A2 synthesis in platelets and reduce fibrin clot formation; the interaction is pharmacodynamic but additive only at the level of platelet inhibition, not at the mucosal barrier

ANSWER: B

Rationale:

This case illustrates a clinically important and frequently encountered pharmacodynamic drug interaction with two distinct but convergent mechanisms of harm operating simultaneously at different levels of gastrointestinal homeostasis. Naproxen is a non-selective COX-1 and COX-2 inhibitor. COX-1 in gastric mucosa generates prostaglandins (PGE2, PGI2) that are cytoprotective: they stimulate mucus and bicarbonate secretion forming the protective mucosal gel layer, maintain mucosal blood flow, and promote epithelial cell renewal. COX-1 inhibition by naproxen reduces prostaglandin synthesis, impairing all of these protective mechanisms and creating a vulnerable gastric mucosa susceptible to acid-peptic injury and ulceration. Once an ulcer forms, effective hemostasis requires normal platelet aggregation (platelet plug formation) and coagulation (fibrin mesh reinforcement). Apixaban, a direct Factor Xa inhibitor, reduces thrombin generation by blocking the prothrombinase complex; reduced thrombin impairs fibrin clot formation and stabilization. At the bleeding ulcer base, the impaired coagulation cascade prevents adequate fibrin clot deposition, allowing sustained arterial bleeding. This is a pharmacodynamic interaction: the two drugs act through entirely different molecular mechanisms (COX-1 inhibition vs Factor Xa inhibition) at entirely different biological sites (gastric mucosa prostaglandin synthesis vs coagulation cascade), yet their combined effect — mucosal vulnerability plus hemostatic impairment — produces hemorrhagic risk far exceeding either drug alone. Acetaminophen (paracetamol) is the appropriate analgesic alternative: it provides analgesia and antipyresis without clinically meaningful COX-1 inhibition in gastric mucosa at therapeutic doses, carries no antiplatelet activity, does not impair prostaglandin-mediated gastric cytoprotection, and does not interact pharmacokinetically with apixaban. Option A is incorrect — naproxen does not inhibit CYP2C9 to a clinically relevant degree; apixaban is primarily metabolized by CYP3A4, not CYP2C9; and ibuprofen shares the same COX-inhibitory gastric toxicity as naproxen. Option C is incorrect — apixaban does not inhibit CYP3A4; the interaction is pharmacodynamic, not pharmacokinetic; and celecoxib, while more COX-2 selective, still carries gastrointestinal and cardiovascular risks in anticoagulated elderly patients. Option D is incorrect — naproxen plays a direct and major pharmacodynamic role in the hemorrhage by impairing gastric cytoprotection; the interaction is not attributable to apixaban alone. Option E is incorrect — naproxen's primary gastrointestinal mechanism is COX-1-mediated impairment of mucosal prostaglandin synthesis, not thromboxane A2 inhibition in platelets; while naproxen does inhibit platelet COX-1 and thromboxane A2, the mucosal cytoprotection mechanism is the dominant factor in ulcer formation.