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Warfarin-amiodarone interaction — USMLE Step 3 MCQ

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ModerateInternal Medicine (chronic management)Warfarin-amiodarone interactionUSMLE Step 3

A 74-year-old man with atrial fibrillation has been stable on warfarin for years. Two weeks after starting amiodarone, routine testing shows INR 4.8. He has no bleeding. Diet has not changed. Which of the following is the most likely explanation for this patient's findings?

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Correct answer: D — Inhibition of warfarin metabolism by amiodarone

Amiodarone inhibits hepatic metabolism of warfarin and commonly increases INR, requiring dose adjustment and closer monitoring.

Reference: CHEST antithrombotic therapy guideline