Antimicrobials USMLE Step 1 Practice Question
A 62-year-old woman with gastroesophageal reflux disease takes daily omeprazole. She is prescribed clarithromycin for H. pylori eradication as part of triple therapy. After 5 days, she develops palpitations, dizziness, and syncope. Vital signs show HR 110 bpm, BP 98/62 mmHg. ECG demonstrates QT prolongation (520 ms). Serum potassium is 3.1 mEq/L. She denies chest pain. Which mechanism best explains this adverse drug interaction?
Answer choices
- AClarithromycin blocks cardiac potassium channels directly, causing QT prolongationCorrect answer
- BCombined therapy causes hypomagnesemia, predisposing to arrhythmias
- COmeprazole increases gastric pH, reducing clarithromycin absorption
- DClarithromycin inhibits CYP3A4, increasing omeprazole levels and causing toxicity
- EOmeprazole induces CYP1A2, decreasing clarithromycin metabolism and causing accumulation
- FClarithromycin competitively inhibits the H+/K+-ATPase pump, causing electrolyte wasting and hypokalemia
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