1. [CASE 1 — QUESTION 1]
A 44-year-old man returns from a 3-month stay in rural India with 2 weeks of right upper quadrant pain, fever, and night sweats. Examination shows tender hepatomegaly. Ultrasound reveals a single 7 cm hypoechoic lesion in the right hepatic lobe, and serology for Entamoeba histolytica is positive. Aspiration is not performed because the clinical and serologic picture is diagnostic of an amebic liver abscess. Which agent is the treatment of choice to clear the invasive infection in the liver?
A) Diloxanide furoate alone
B) Sodium stibogluconate
C) Metronidazole
D) Suramin
ANSWER: C
Rationale:
Metronidazole is the tissue agent of choice for amebic liver abscess; it reaches high concentrations in hepatic tissue and rapidly kills invasive trophozoites, producing clinical improvement.
Option A: Option A is incorrect: diloxanide furoate is a luminal agent that clears intestinal cysts but does not treat invasive tissue disease, so it cannot be used alone for the abscess.
Option B: Option B is incorrect: sodium stibogluconate is a pentavalent antimonial for leishmaniasis and has no role in amebiasis.
Option D: Option D is incorrect: suramin is an antitrypanosomal agent for stage 1 African trypanosomiasis and is not active against Entamoeba histolytica.
2. [CASE 1 — QUESTION 2]
Continuing with the same patient. The team discusses how metronidazole achieves selective killing of the parasite without harming the patient's own cells. Which mechanism underlies this selectivity?
A) Low-redox-potential electron donors in the organism (ferredoxin and pyruvate:ferredoxin oxidoreductase) reduce the nitro group to a reactive radical that damages parasite DNA, while host cells lack these donors
B) The drug inhibits the parasite 50S ribosomal subunit, which differs structurally from the host ribosome
C) The drug is concentrated selectively in hepatic tissue by an active transporter unique to the abscess cavity
D) The drug chelates iron required only by the parasite
ANSWER: A
Rationale:
Metronidazole is a prodrug that must be reductively activated. Anaerobic and microaerophilic organisms possess low-redox-potential electron donors (ferredoxin; pyruvate:ferredoxin oxidoreductase) that reduce the nitro group to a reactive radical anion, which breaks parasite DNA; aerobic host cells lack these donors, so the drug is not activated in host tissue.
Option B: Option B is incorrect: metronidazole acts on DNA, not the ribosome, and is not a protein-synthesis inhibitor.
Option C: Option C is incorrect: selectivity arises from the activation requirement, not an abscess-specific transporter.
Option D: Option D is incorrect: the mechanism is reductive radical generation, not iron chelation.
3. [CASE 1 — QUESTION 3]
Continuing with the same patient. He responds well to metronidazole, and his fever and pain resolve. To prevent relapse and ongoing transmission, what additional treatment is required after the metronidazole course?
A) A second 10-day course of metronidazole
B) Addition of an aerobic gram-negative antibiotic
C) Long-term corticosteroids to suppress residual inflammation
D) A luminal agent such as diloxanide furoate or iodoquinol to eradicate intestinal cyst carriage
ANSWER: D
Rationale:
Metronidazole clears invasive trophozoites in tissue but does not reliably eradicate Entamoeba histolytica cysts in the intestinal lumen; those cysts are the reservoir for relapse and transmission, so a luminal agent (diloxanide furoate or iodoquinol) must follow the tissue course.
Option A: Option A is incorrect: repeating tissue-directed therapy does not address the luminal cysts and will not prevent relapse.
Option B: Option B is incorrect: amebic liver abscess is not a polymicrobial pyogenic process, so aerobic gram-negative coverage is not indicated.
Option C: Option C is incorrect: corticosteroids are not part of standard amebic abscess management and may be harmful.
4. [CASE 1 — QUESTION 4]
Continuing with the same patient. On day 3 of metronidazole he attends a celebration and drinks two beers, then develops facial flushing, throbbing headache, nausea, vomiting, and palpitations. Which mechanism explains this reaction?
A) An immunoglobulin E-mediated allergic response to metronidazole
B) Inhibition of acetaldehyde dehydrogenase by metronidazole, causing acetaldehyde to accumulate after alcohol ingestion
C) Inhibition of alcohol dehydrogenase, blocking conversion of ethanol to acetaldehyde
D) Induction of CYP2E1, accelerating ethanol oxidation
ANSWER: B
Rationale:
Metronidazole inhibits acetaldehyde dehydrogenase, the enzyme that clears acetaldehyde produced from ethanol; acetaldehyde then accumulates and produces the flushing, headache, nausea, vomiting, and palpitations of a disulfiram-like reaction. Patients must avoid alcohol during therapy and for 48 hours afterward (72 hours for tinidazole).
Option A: Option A is incorrect: this is a metabolic acetaldehyde reaction, not an immunoglobulin E-mediated allergy.
Option C: Option C is incorrect: inhibiting alcohol dehydrogenase would reduce acetaldehyde formation, the opposite of what occurs.
Option D: Option D is incorrect: the reaction results from impaired acetaldehyde clearance, not from accelerated ethanol oxidation via CYP2E1 induction.
5. [CASE 2 — QUESTION 1]
A 29-year-old aid worker returns from Guinea with several weeks of intermittent fever, posterior cervical lymphadenopathy, and progressive daytime sleepiness with disturbed nighttime sleep. A blood smear shows trypomastigotes consistent with Trypanosoma brucei gambiense. Before any drug is selected, which step is mandatory and why?
A) Begin empiric pentamidine immediately to avoid treatment delay
B) Perform a lumbar puncture, because cerebrospinal fluid findings determine the stage and therefore which therapy is required
C) Obtain a 24-hour urine protein collection to grade baseline renal function
D) Start melarsoprol presumptively because central nervous system symptoms are present
ANSWER: B
Rationale:
Human African trypanosomiasis is staged by cerebrospinal fluid examination (pleocytosis above 5 white blood cells per microliter or trypanosomes in cerebrospinal fluid), and stage determines whether a central-nervous-system-penetrating drug is required; lumbar puncture is therefore mandatory before treatment selection.
Option A: Option A is incorrect: pentamidine treats stage 1 gambiense, and giving it before staging risks undertreating undetected stage 2 disease.
Option C: Option C is incorrect: urine protein collection does not stage the disease, though urinalysis is monitored during suramin therapy.
Option D: Option D is incorrect: melarsoprol carries a high risk of fatal encephalopathy and is reserved for confirmed stage 2 disease, not given presumptively on symptoms alone.
6. [CASE 2 — QUESTION 2]
Continuing with the same patient. Lumbar puncture shows 30 white blood cells per microliter with trypanosomes in the cerebrospinal fluid, confirming stage 2 disease. A colleague suggests suramin. Why would suramin fail in this patient?
A) Suramin does not cross the blood-brain barrier, so it cannot reach parasites established in the central nervous system
B) Suramin is active only against Trypanosoma cruzi, not African subspecies
C) Suramin requires reductive activation that occurs only in cerebrospinal fluid
D) Suramin is rapidly inactivated by the high protein content of cerebrospinal fluid
ANSWER: A
Rationale:
Suramin binds extensively to plasma proteins and does not cross the blood-brain barrier; it is effective only in stage 1 disease, so in confirmed stage 2 infection it cannot reach central nervous system parasites and would fail.
Option B: Option B is incorrect: suramin treats stage 1 African trypanosomiasis (notably Trypanosoma brucei rhodesiense); it is not a Trypanosoma cruzi agent.
Option C: Option C is incorrect: suramin is not a prodrug requiring reductive activation.
Option D: Option D is incorrect: the limitation is failure to cross the blood-brain barrier, not inactivation within cerebrospinal fluid.
7. [CASE 2 — QUESTION 3]
Continuing with the same patient. Given confirmed stage 2 Trypanosoma brucei gambiense disease, which regimen is the current standard of care?
A) Suramin followed by pentamidine
B) Melarsoprol monotherapy as first choice
C) Single-dose intramuscular pentamidine
D) Nifurtimox-eflornithine combination therapy
ANSWER: D
Rationale:
Nifurtimox-eflornithine combination therapy is the standard of care for stage 2 Trypanosoma brucei gambiense; it pairs oral nifurtimox with a shortened eflornithine course, matching the efficacy of eflornithine monotherapy while reducing drug burden and improving feasibility.
Option A: Option A is incorrect: both suramin and pentamidine are stage 1 agents and do not treat central nervous system disease.
Option B: Option B is incorrect: melarsoprol is now largely replaced for gambiense stage 2 by better-tolerated options because of its high reactive-encephalopathy risk.
Option C: Option C is incorrect: pentamidine treats stage 1 gambiense only and does not reliably reach the central nervous system.
8. [CASE 2 — QUESTION 4]
Continuing with the same patient. The eflornithine component irreversibly inhibits ornithine decarboxylase. This drug is effective against gambiense but not against Trypanosoma brucei rhodesiense. What explains this subspecies selectivity?
A) Eflornithine reaches therapeutic cerebrospinal fluid levels only in gambiense infection
B) Rhodesiense expresses an ornithine decarboxylase that eflornithine cannot bind
C) Gambiense turns ornithine decarboxylase over slowly, so the irreversible block persists and is lethal, whereas rhodesiense rapidly resynthesizes the enzyme and recovers
D) Rhodesiense bypasses polyamine synthesis entirely through an alternative pathway
ANSWER: C
Rationale:
With an irreversible inhibitor, the duration of effect depends on how fast new enzyme is made. Gambiense turns ornithine decarboxylase over slowly, so the block persists long enough to halt polyamine synthesis and kill the parasite; rhodesiense resynthesizes the enzyme quickly and escapes.
Option A: Option A is incorrect: eflornithine does penetrate the central nervous system, and the difference is enzyme turnover, not differential cerebrospinal fluid levels.
Option B: Option B is incorrect: the enzyme is bound in both subspecies; the difference is replacement rate.
Option D: Option D is incorrect: both subspecies depend on polyamine synthesis, so there is no bypass pathway in rhodesiense.
9. [CASE 3 — QUESTION 1]
A 9-year-old child in Bolivia presents with fever, unilateral periorbital swelling (Romana sign), and regional lymphadenopathy. A blood smear demonstrates Trypanosoma cruzi trypomastigotes, confirming acute Chagas disease. What is the appropriate first-line treatment and expected outcome?
A) Benznidazole, with high parasitological cure rates expected in acute and pediatric disease
B) Suramin, the agent of choice for acute American trypanosomiasis
C) Deferral of treatment until the chronic indeterminate phase
D) Melarsoprol, because of the risk of central nervous system spread
ANSWER: A
Rationale:
Acute Chagas disease should be treated promptly with benznidazole, the first-line agent; cure rates exceed 80 percent in the acute phase, and children tolerate the drug better than adults.
Option B: Option B is incorrect: suramin treats African trypanosomiasis, not Chagas disease.
Option C: Option C is incorrect: deferring treatment forgoes the highest-cure window, so acute disease is treated immediately.
Option D: Option D is incorrect: melarsoprol is an African trypanosomiasis agent and is not used for Chagas disease.
10. [CASE 3 — QUESTION 2]
Continuing with the same patient. The child begins a 60-day benznidazole course. Which monitoring is most important to detect a dose-limiting hematologic toxicity of benznidazole?
A) Serial blood glucose before and after each dose
B) Weekly electrocardiograms to detect QTc prolongation
C) Complete blood count, classically at weeks 2, 4, and 8, to detect bone marrow suppression
D) Serial urinalysis for proteinuria
ANSWER: C
Rationale:
Benznidazole can cause bone marrow suppression (leukopenia), so a complete blood count is monitored during therapy, classically at weeks 2, 4, and 8; an absolute neutrophil count below 1,500 cells/mm3 warrants dose reduction or interruption. Dermatologic reactions and peripheral neuropathy are its other major adverse effects.
Option A: Option A is incorrect: glucose monitoring before and after dosing applies to pentamidine, not benznidazole.
Option B: Option B is incorrect: QTc monitoring is required for pentavalent antimonials, not benznidazole.
Option D: Option D is incorrect: pre-dose urinalysis for proteinuria is the monitoring rule for suramin nephrotoxicity, not benznidazole.
11. [CASE 3 — QUESTION 3]
Continuing with the same patient. Suppose the child could not tolerate benznidazole and nifurtimox were substituted. Which statement correctly pairs nifurtimox's mechanism with its clinical profile?
A) It inhibits ornithine decarboxylase and chiefly causes bone marrow suppression
B) It undergoes one-electron reduction to generate reactive oxygen species that overwhelm the parasite trypanothione defense, and it is a second-line agent with prominent neuropsychiatric and gastrointestinal adverse effects
C) It binds ergosterol-like membrane sterols and chiefly causes infusion-related nephrotoxicity
D) It inhibits dihydrofolate reductase and chiefly causes megaloblastic anemia
ANSWER: B
Rationale:
Nifurtimox is a nitrofuran that undergoes one-electron reduction to superoxide and other reactive oxygen species, overwhelming the trypanothione-based antioxidant defense of Trypanosoma cruzi; it is used second-line when benznidazole is not tolerated and has a less favorable profile, with prominent neuropsychiatric effects (insomnia, irritability, polyneuropathy) and gastrointestinal upset.
Option A: Option A is incorrect: ornithine decarboxylase inhibition describes eflornithine.
Option C: Option C is incorrect: binding ergosterol-like sterols describes amphotericin B.
Option D: Option D is incorrect: dihydrofolate reductase inhibition describes pyrimethamine.
12. [CASE 3 — QUESTION 4]
Continuing with the same patient. During a family visit, the child's 55-year-old grandfather, who has established Chagas cardiomyopathy, is also seen, and the family asks whether benznidazole will protect his heart. Citing the BENEFIT trial, which statement is most accurate?
A) Benznidazole reverses established cardiomyopathy and is strongly indicated for his heart disease
B) Benznidazole increases cardiac mortality in chronic disease and is contraindicated
C) Benznidazole has no antiparasitic activity in the chronic phase, so it is pointless
D) Benznidazole reduces polymerase chain reaction detection of the parasite but did not reduce cardiac events or mortality over 5 years in patients with established cardiomyopathy, so the cardiac benefit in advanced disease is uncertain
ANSWER: D
Rationale:
In the BENEFIT trial, benznidazole lowered polymerase chain reaction detection of Trypanosoma cruzi but did not reduce cardiac events or mortality over 5 years in patients with established Chagas cardiomyopathy; the benefit of treating advanced cardiac disease is therefore uncertain, even though younger patients with indeterminate or early disease are still treated.
Option A: Option A is incorrect: the trial did not show reversal of established cardiomyopathy.
Option B: Option B is incorrect: it did not demonstrate increased cardiac mortality or establish a contraindication.
Option C: Option C is incorrect: the reduction in parasite detection shows the drug does have antiparasitic activity; it simply did not improve cardiac outcomes in this group.
13. [CASE 4 — QUESTION 1]
A 24-year-old man from Bihar, India, presents with months of fever, weight loss, and massive splenomegaly. Bone marrow aspirate shows Leishmania amastigotes, confirming visceral leishmaniasis. For an immunocompetent patient in this setting, which agent is the World Health Organization-preferred first-line treatment, and what is its mechanism?
A) Sodium stibogluconate, which reduces to trivalent antimony and inhibits trypanothione reductase
B) Pentamidine, which accumulates in the kinetoplast and disrupts kinetoplast DNA
C) Liposomal amphotericin B, which binds ergosterol-like membrane sterols and concentrates in the reticuloendothelial system where amastigotes reside
D) Suramin, which inhibits parasite glycolytic enzymes
ANSWER: C
Rationale:
Liposomal amphotericin B is the treatment of choice for visceral leishmaniasis in most settings; it binds ergosterol-like sterols in the Leishmania membrane and concentrates in the reticuloendothelial system (liver, spleen, marrow), exactly where amastigotes reside within macrophages, while limiting nephrotoxicity.
Option A: Option A is incorrect: antimonials are limited by high regional resistance in Bihar and are not WHO-preferred there.
Option B: Option B is incorrect: pentamidine is a second-line agent for visceral leishmaniasis, not the preferred first-line drug.
Option D: Option D is incorrect: suramin is an antitrypanosomal agent with no role in leishmaniasis.
14. [CASE 4 — QUESTION 2]
Continuing with the same patient. A trainee asks why pentavalent antimonials, once the mainstay, are not chosen here. What is the best explanation?
A) Antimonials are effective but require a central line, which is unavailable
B) In the Bihar region, antimonial resistance in Leishmania donovani has reached 60 percent or more, rendering these agents effectively obsolete for South Asian visceral leishmaniasis
C) Antimonials are contraindicated in any patient under age 30
D) Antimonials cannot treat visceral disease and work only in cutaneous leishmaniasis
ANSWER: B
Rationale:
In Bihar and neighboring areas, antimonial resistance in Leishmania donovani is extremely high (60 percent or more), so these drugs are effectively obsolete for South Asian visceral leishmaniasis; they retain utility in East African, Mediterranean, and South American settings.
Option A: Option A is incorrect: the limitation is resistance, not vascular access.
Option C: Option C is incorrect: there is no blanket age contraindication under 30.
Option D: Option D is incorrect: antimonials do treat visceral leishmaniasis in regions without high resistance; the issue here is regional resistance.
15. [CASE 4 — QUESTION 3]
Continuing with the same patient. Before starting therapy he is counseled that, had an oral agent such as miltefosine been chosen, specific precautions would apply — and the team reviews the counseling that would be essential for any patient of childbearing potential receiving miltefosine. Which counseling point is essential?
A) Miltefosine is teratogenic (Category X) with a long half-life, so a negative pregnancy test must be confirmed and effective contraception used during treatment and for 5 months afterward
B) Miltefosine must be given on an empty stomach and causes a disulfiram-like reaction with alcohol
C) Miltefosine requires weekly electrocardiograms because it prolongs the QTc interval
D) Miltefosine is safe in pregnancy and requires no contraceptive precautions
ANSWER: A
Rationale:
Miltefosine is teratogenic (Category X), and its long half-life (about 7 days, with drug detectable more than 5 weeks after the last dose) means teratogenic exposure persists beyond the course; a negative pregnancy test must be confirmed and effective contraception used during treatment and for 5 months afterward.
Option B: Option B is incorrect: miltefosine is better tolerated with food, and the disulfiram-like reaction is a nitroimidazole effect, not a miltefosine effect.
Option C: Option C is incorrect: QTc monitoring applies to pentavalent antimonials, not miltefosine.
Option D: Option D is incorrect and dangerous: miltefosine is contraindicated in pregnancy and mandates contraceptive precautions.
16. [CASE 4 — QUESTION 4]
Continuing with the same patient. He is found to have advanced HIV (CD4 40 cells/mm3), and over the following year his visceral leishmaniasis relapses twice despite completing liposomal amphotericin B each time. Besides optimizing antiretroviral therapy, what is the most appropriate strategy?
A) Permanently switch to sodium stibogluconate monotherapy
B) Stop all antileishmanial therapy after each acute episode resolves
C) Give a single oral dose of metronidazole at each relapse
D) Begin secondary prophylaxis with periodic (for example, monthly) liposomal amphotericin B and continue until immune reconstitution
ANSWER: D
Rationale:
In HIV-coinfected visceral leishmaniasis, relapse rates approach 100 percent while immunosuppression persists, so secondary prophylaxis with periodic liposomal amphotericin B (commonly monthly) is recommended, continued until immune reconstitution on antiretroviral therapy.
Option A: Option A is incorrect: antimonial monotherapy is limited by resistance and toxicity and is not the maintenance strategy of choice.
Option B: Option B is incorrect: stopping therapy after each episode is what permits recurrent relapse; ongoing prophylaxis is needed.
Option C: Option C is incorrect: metronidazole has no activity against Leishmania.
17. [CASE 5 — QUESTION 1]
A 39-year-old man with untreated HIV (CD4 25 cells/mm3) presents with headache, left hemiparesis, and confusion. Brain magnetic resonance imaging shows multiple ring-enhancing lesions with edema, and Toxoplasma gondii serology is positive. Which regimen is first-line?
A) Liposomal amphotericin B plus flucytosine
B) Metronidazole plus a luminal agent
C) Benznidazole plus nifurtimox
D) Pyrimethamine plus sulfadiazine plus folinic acid
ANSWER: D
Rationale:
This is classic toxoplasmic encephalitis in advanced HIV (multiple ring-enhancing lesions, positive serology, CD4 below 100); first-line therapy is pyrimethamine plus sulfadiazine with folinic acid (leucovorin) to prevent pyrimethamine marrow toxicity.
Option A: Option A is incorrect: amphotericin-flucytosine treats cryptococcal disease, not toxoplasmosis.
Option B: Option B is incorrect: metronidazole plus a luminal agent treats amebiasis, not cerebral toxoplasmosis.
Option C: Option C is incorrect: benznidazole and nifurtimox treat Chagas disease.
18. [CASE 5 — QUESTION 2]
Continuing with the same patient. Folinic acid is added to the regimen. A trainee asks why folinic acid, and not folic acid, must be used. What is the correct explanation?
A) Folinic acid is already-reduced folate that bypasses the dihydrofolate reductase block in host cells, protecting marrow, whereas folic acid is the oxidized form that still requires dihydrofolate reductase and cannot bypass the block
B) Folinic acid inhibits dihydropteroate synthase in the parasite, deepening the blockade
C) Folinic acid increases renal clearance of pyrimethamine, lowering its toxicity
D) Folinic acid and folic acid are interchangeable, so either may be used
ANSWER: A
Rationale:
Pyrimethamine blocks dihydrofolate reductase, so the rescue agent must supply folate that does not need that enzyme. Folinic acid is already-reduced folate that host cells use directly, bypassing the block and preventing marrow toxicity, while the parasite, which synthesizes folate de novo, remains susceptible. Folic acid is the oxidized form that still requires dihydrofolate reductase, so it cannot bypass the block and must not be substituted.
Option B: Option B is incorrect: folinic acid supplies reduced folate and does not inhibit dihydropteroate synthase.
Option C: Option C is incorrect: the protection is metabolic bypass, not altered pyrimethamine clearance.
Option D: Option D is incorrect and is the dangerous misconception this question targets: the two are not interchangeable.
19. [CASE 5 — QUESTION 3]
Continuing with the same patient. After 2 weeks of appropriate therapy he shows no clinical or radiologic improvement. What is the most appropriate next step?
A) Double the sulfadiazine dose and continue for another month without further evaluation
B) Switch to metronidazole, since the lesions are likely amebic
C) Pursue brain biopsy to exclude an alternative diagnosis such as primary central nervous system lymphoma, which can mimic toxoplasmic encephalitis on imaging
D) Stop all therapy, since lack of response proves the lesions are benign
ANSWER: C
Rationale:
Toxoplasmic encephalitis is expected to respond clinically and radiologically within about 2 weeks; failure to improve should prompt brain biopsy to exclude alternative diagnoses, particularly primary central nervous system lymphoma, which can closely mimic toxoplasmic encephalitis on imaging in patients with HIV.
Option A: Option A is incorrect: blindly escalating therapy without re-evaluation delays diagnosis of a possible lymphoma.
Option B: Option B is incorrect: these are not amebic lesions, and metronidazole is not indicated.
Option D: Option D is incorrect: lack of response is a red flag for an alternative diagnosis, not evidence the lesions are benign.
20. [CASE 5 — QUESTION 4]
Continuing with the same patient. As his case is reviewed, the team contrasts his management with that of a pregnant patient: a 31-year-old woman at 10 weeks gestation with acute Toxoplasma gondii seroconversion and no current fetal infection on amniocentesis and imaging. Which agent is most appropriate to reduce the risk of transmission to her fetus?
A) Pyrimethamine plus sulfadiazine plus folinic acid as definitive fetal treatment
B) Spiramycin, which concentrates in placental tissue and reduces vertical transmission
C) Metronidazole to clear the maternal infection
D) Benznidazole to interrupt transmission
ANSWER: B
Rationale:
When maternal infection is confirmed but the fetus is not yet infected, spiramycin is used because it concentrates in placental tissue and reduces vertical transmission, without the first-trimester teratogenic concern of pyrimethamine.
Option A: Option A is incorrect: pyrimethamine-sulfadiazine-folinic acid is reserved for confirmed fetal infection and pyrimethamine is generally avoided in the first trimester, so it is not used here for transmission prophylaxis.
Option C: Option C is incorrect: metronidazole has no activity against Toxoplasma.
Option D: Option D is incorrect: benznidazole treats Chagas disease and has no role in toxoplasmosis.
21. [CASE 6 — QUESTION 1]
A 30-year-old woman presents with frothy yellow-green vaginal discharge and dysuria. Wet mount shows motile flagellated organisms, and a nucleic acid amplification test confirms Trichomonas vaginalis. Which agent is first-line therapy?
A) Liposomal amphotericin B
B) Metronidazole (or single-dose tinidazole)
C) Suramin
D) Pyrimethamine plus sulfadiazine
ANSWER: B
Rationale:
Trichomoniasis is treated with a nitroimidazole; metronidazole is first-line, and single-dose tinidazole is an effective alternative that improves adherence.
Option A: Option A is incorrect: liposomal amphotericin B treats leishmaniasis, not trichomoniasis.
Option C: Option C is incorrect: suramin is an antitrypanosomal agent and has no activity against Trichomonas.
Option D: Option D is incorrect: pyrimethamine-sulfadiazine treats toxoplasmosis, not trichomoniasis.
22. [CASE 6 — QUESTION 2]
Continuing with the same patient. She has difficulty adhering to multiday regimens and asks about a single-dose option. Which property of tinidazole supports single-dose therapy, and what counseling follows?
A) Tinidazole is active against aerobes, so a single dose clears mixed infection
B) Tinidazole has no alcohol interaction, so no counseling is needed
C) Tinidazole is the only nitroimidazole that crosses the blood-brain barrier, allowing a single dose
D) Tinidazole has a longer half-life (about 12 to 14 hours) that supports effective single-dose therapy, and because of that longer half-life alcohol must be avoided for about 72 hours after the dose
ANSWER: D
Rationale:
Tinidazole's longer half-life (about 12 to 14 hours, versus roughly 6 to 10 hours for metronidazole) and better gastrointestinal tolerability allow effective single-dose therapy; the same longer half-life means alcohol must be avoided for about 72 hours after the dose to prevent a disulfiram-like reaction.
Option A: Option A is incorrect: tinidazole acts on anaerobes and microaerophiles, not aerobes.
Option B: Option B is incorrect: tinidazole also causes a disulfiram-like reaction, so alcohol counseling is required.
Option C: Option C is incorrect: metronidazole itself penetrates the central nervous system well, so this is not a distinguishing tinidazole property.
23. [CASE 6 — QUESTION 3]
Continuing with the same patient. She is also taking chronic warfarin, and after starting metronidazole she returns with an international normalized ratio rising from 2.5 to 6.5 and new bruising. Which mechanism explains this?
A) Metronidazole inhibits CYP2C9, reducing clearance of the potent S-enantiomer of warfarin and potentiating its anticoagulant effect
B) Metronidazole induces CYP2C9, increasing the international normalized ratio paradoxically
C) Metronidazole displaces vitamin K from hepatic stores, enhancing clotting-factor synthesis
D) Metronidazole increases gut absorption of warfarin by chelation
ANSWER: A
Rationale:
Metronidazole inhibits CYP2C9, the enzyme that clears the more potent S-enantiomer of warfarin; reduced clearance raises warfarin exposure, elevates the international normalized ratio, and increases bleeding risk, so the international normalized ratio must be monitored and warfarin adjusted.
Option B: Option B is incorrect: metronidazole inhibits rather than induces CYP2C9, and induction would lower the international normalized ratio.
Option C: Option C is incorrect: metronidazole does not mobilize vitamin K to enhance clotting-factor synthesis, which would oppose the observed rise.
Option D: Option D is incorrect: the interaction is hepatic enzyme inhibition, not gut chelation enhancing absorption.
24. [CASE 6 — QUESTION 4]
Continuing with the same patient. After completing therapy she still has symptoms (returning to the original trichomoniasis patient). She admits she stopped the drug early after a reaction to wine. Before invoking drug resistance, what is the most appropriate interpretation and next step?
A) This is high-level metronidazole resistance; begin desensitization immediately
B) This indicates a fungal coinfection; add fluconazole
C) The most likely explanation is incomplete therapy from nonadherence (driven by the disulfiram-like reaction); reinforce alcohol avoidance and re-treat with a complete course, reserving resistance testing and higher-dose or alternative regimens for confirmed failure
D) The symptoms confirm cure; no further action is needed
ANSWER: C
Rationale:
True metronidazole resistance in Trichomonas vaginalis exists but is uncommon (roughly 2 to 5 percent low-level); persistent symptoms after an interrupted course are far more likely to reflect nonadherence, here driven by a disulfiram-like reaction to alcohol. The appropriate step is to reinforce alcohol avoidance and complete a full course, reserving resistance work-up and higher-dose or alternative regimens for confirmed failure after adequate adherent treatment.
Option A: Option A is incorrect: jumping to desensitization for presumed high-level resistance is premature when nonadherence is the likely cause.
Option B: Option B is incorrect: the wet mount and testing confirmed Trichomonas, not a fungal infection.
Option D: Option D is incorrect: persistent symptoms indicate the infection is not cured, so action is required.
25. [CASE 7 — QUESTION 1]
A 33-year-old safari guide from East Africa presents with an acute febrile illness, a trypanosomal chancre, and rapid neurologic deterioration. Blood and cerebrospinal fluid confirm stage 2 Trypanosoma brucei rhodesiense infection. Which agent is appropriate for stage 2 rhodesiense disease?
A) Melarsoprol
B) Eflornithine monotherapy
C) Suramin
D) Pentamidine
ANSWER: A
Rationale:
Stage 2 Trypanosoma brucei rhodesiense disease requires a central-nervous-system-penetrating agent, and melarsoprol is the treatment used for stage 2 rhodesiense.
Option B: Option B is incorrect: eflornithine is selective for Trypanosoma brucei gambiense (because of slow ornithine decarboxylase turnover) and is ineffective against rhodesiense.
Option C: Option C is incorrect: suramin is a stage 1 agent and does not cross the blood-brain barrier.
Option D: Option D is incorrect: pentamidine treats stage 1 gambiense and does not reliably treat central nervous system disease.
26. [CASE 7 — QUESTION 2]
Continuing with the same patient. By what mechanism does melarsoprol kill the trypanosome?
A) It irreversibly inhibits ornithine decarboxylase, depleting polyamines
B) It generates reactive oxygen species through one-electron reduction
C) Its trivalent arsenic moiety reacts with trypanothione and inhibits trypanothione reductase, collapsing the parasite redox defenses
D) It binds ergosterol-like membrane sterols, causing lethal ion flux
ANSWER: C
Rationale:
Melarsoprol is an organoarsenical whose trivalent arsenic reacts with trypanothione (the parasite-specific glutathione-spermidine conjugate) and inhibits trypanothione reductase, collapsing the redox homeostasis the parasite depends on.
Option A: Option A is incorrect: irreversible ornithine decarboxylase inhibition describes eflornithine.
Option B: Option B is incorrect: reactive-oxygen-species generation by one-electron reduction describes nifurtimox.
Option D: Option D is incorrect: binding ergosterol-like sterols describes amphotericin B.
27. [CASE 7 — QUESTION 3]
Continuing with the same patient. On day 4 of melarsoprol he develops fever, rapidly declining consciousness, and a seizure. Which complication is most likely, and what adjunct reduces its risk?
A) Hypoglycemia from beta-cell toxicity; intravenous dextrose prevents it
B) Post-treatment reactive encephalopathy, occurring in roughly 5 to 10 percent of patients and fatal in about half of those affected; concurrent prednisolone reduces but does not eliminate the risk
C) Torsades de pointes; intravenous magnesium prevents it
D) Acute pancreatitis; bowel rest reduces its risk
ANSWER: B
Rationale:
Melarsoprol causes post-treatment reactive encephalopathy in about 5 to 10 percent of patients, an inflammatory syndrome fatal in roughly half of those affected, presenting with fever, depressed consciousness, and seizures during treatment; concurrent prednisolone (1 mg/kg/day) reduces but does not eliminate the risk.
Option A: Option A is incorrect: hypoglycemia from beta-cell toxicity is a pentamidine effect.
Option C: Option C is incorrect: torsades de pointes is a pentavalent antimonial concern and would present as arrhythmia, not encephalopathy.
Option D: Option D is incorrect: pancreatitis is an antimonial toxicity and does not explain this neurologic picture.
28. [CASE 7 — QUESTION 4]
Continuing with the same patient. As his care team reviews trypanosomiasis drug safety, a trainee is asked to match each agent to its key pre-dose safety check. Which pairing is correct?
A) Melarsoprol requires a pre-dose urinalysis, and proteinuria above 2+ halts melarsoprol dosing
B) Suramin requires pre-dose neurologic checks for reactive encephalopathy and concurrent prednisolone
C) Eflornithine requires pre-dose blood glucose because it causes hypoglycemia
D) Suramin requires a pre-dose urinalysis (proteinuria above 2+ contraindicates further dosing because of nephrotoxicity), whereas melarsoprol requires close neurologic monitoring for reactive encephalopathy with prednisolone co-administration
ANSWER: D
Rationale:
The two agents have distinct safety checks. Suramin is nephrotoxic, so urinalysis is performed before each dose and proteinuria above 2+ contraindicates further dosing; melarsoprol's defining hazard is post-treatment reactive encephalopathy, managed with neurologic monitoring and prednisolone co-administration.
Option A: Option A is incorrect: the urinalysis-and-proteinuria rule belongs to suramin, not melarsoprol.
Option B: Option B is incorrect: reactive encephalopathy and prednisolone belong to melarsoprol, not suramin.
Option C: Option C is incorrect: pre-dose glucose monitoring for hypoglycemia belongs to pentamidine, not eflornithine.
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