Antimicrobials USMLE Step 1 Practice Question
A 72-year-old woman with a CD4 count of 45 cells/μL presents to the emergency department with palpitations and dyspnea on exertion. She was started on azithromycin 1200 mg weekly 5 days ago for Mycobacterium avium complex (MAC) prophylaxis. Her current medications include digoxin 0.25 mg daily for systolic heart failure. Vital signs: HR 128 bpm and irregular, BP 98/62 mmHg, RR 22/min, temperature 37.1°C. Serum digoxin level is 3.2 ng/mL (therapeutic range 0.8–2.0 ng/mL). Serum potassium is 4.1 mEq/L, serum creatinine is 0.9 mg/dL, and serum magnesium is 2.0 mg/dL. Electrocardiography reveals atrial fibrillation with a rapid ventricular response and nonspecific ST changes. Which of the following best explains the elevated serum digoxin concentration?
Answer choices
- AAzithromycin-induced hypokalemia and hypomagnesemia potentiating digoxin cardiotoxicity
- BInhibition of P-glycoprotein-mediated renal tubular secretion of digoxinCorrect answer
- CInhibition of cytochrome P450 3A4-mediated hepatic metabolism of digoxin
- DIncreased digoxin absorption due to azithromycin-induced gastric pH elevation
- EAzithromycin metabolite-mediated direct inhibition of Na+/K+-ATPase activity
- FDecreased renal perfusion secondary to azithromycin-induced acute kidney injury
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