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Elevated INR on antibiotics — USMLE Step 3 MCQ

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ModerateInternal Medicine (chronic management)Elevated INR on antibioticsUSMLE Step 3

A 76-year-old man on stable warfarin for atrial fibrillation starts trimethoprim-sulfamethoxazole for cellulitis. Five days later his INR is 5.8 without bleeding. Diet and warfarin dose have not changed. Which of the following is the most likely explanation for this patient's findings?

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Correct answer: E — Inhibition of warfarin metabolism by the antibiotic

Trimethoprim-sulfamethoxazole can increase warfarin effect through metabolic interaction and reduced vitamin K-producing flora.

Reference: Anticoagulation management guidance: https://www.acc.org/