Antimicrobials USMLE Step 1 Practice Question
A 67-year-old woman with atrial fibrillation presents with dysuria, frequency, and suprapubic pain. Vitals: BP 138/82, HR 92, RR 16, Temp 38.2°C, SpO2 98%. Urinalysis shows pyuria and bacteriuria; urine culture pending. She takes warfarin (INR 2.3 baseline) for anticoagulation. Ciprofloxacin 500 mg orally twice daily is initiated for presumed UTI. Within one week, INR increases to 4.8 without dietary changes. Which mechanism best explains this drug interaction?
Answer choices
- AFluoroquinolones enhance hepatic production of vitamin K-dependent clotting factors
- BCiprofloxacin inhibits CYP2C9, reducing warfarin metabolismCorrect answer
- CCiprofloxacin inhibits warfarin protein binding, increasing free warfarin levels
- DCiprofloxacin increases warfarin absorption through enhanced GI motility
- ECiprofloxacin displaces warfarin from renal tubular reabsorption sites
- FCiprofloxacin inhibits CYP3A4, which is responsible for warfarin glucuronidation and excretion
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