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Warfarin drug interaction — USMLE Step 3 MCQ

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HardInternal Medicine (chronic management)Warfarin drug interactionUSMLE Step 3

A 64-year-old woman with atrial fibrillation takes warfarin. She is treated for pneumonia with trimethoprim-sulfamethoxazole by urgent care. Four days later she has epistaxis and easy bruising. INR is 6.8, hemoglobin is stable, and she is hemodynamically stable. 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 trimethoprim-sulfamethoxazole

Trimethoprim-sulfamethoxazole can potentiate warfarin by inhibiting metabolism and altering vitamin K-producing flora, causing supratherapeutic INR and bleeding.

Reference: ACCP antithrombotic therapy guidance; FDA warfarin prescribing information