American Board of Internal Medicine (ABIM) Certification Practice Exam

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What daily supplement can help stabilize fluctuating INRs in patients on warfarin?

Iron supplements
Vitamin D
Vitamin K
Omega-3 fatty acids

The correct answer is: Vitamin K

Warfarin is an anticoagulant that works by inhibiting the vitamin K-dependent synthesis of certain clotting factors essential for blood coagulation. Because vitamin K plays a pivotal role in this process, fluctuations in vitamin K intake can lead to unstable International Normalized Ratios (INRs) in patients on warfarin therapy. Supplementing with vitamin K can help stabilize INR levels by ensuring a more consistent availability of this vitamin in the diet, which balances the anticoagulant effects of warfarin. It is essential, however, for patients to maintain a consistent intake of vitamin K rather than to change their consumption drastically, as this could lead to significant fluctuations in INR values. Iron supplements, vitamin D, and omega-3 fatty acids do not have a direct impact on warfarin metabolism or the mechanism of action as vitamin K does, making them less relevant in the context of stabilizing INR levels for patients on this medication.