Warfarin inhibits which enzyme as part of its anticoagulant effect?

• A. Inhibits thrombin.
• B. Inhibits vitamin K epoxide reductase, reducing synthesis of vitamin K–dependent clotting factors II, VII, IX, X and proteins C and S.
• C. Activates plasmin.
• D. Inhibits platelet aggregation by COX-1.

Answer: (B)

Warfarin exerts its effect by blocking the vitamin K cycle, specifically inhibiting vitamin K epoxide reductase. This stops the regeneration of reduced vitamin K, which is necessary for the gamma-carboxylation of certain clotting factors and related proteins. As a result, factors II, VII, IX, and X, along with the anticoagulant proteins C and S, are produced in a form that cannot effectively participate in the coagulation cascade. The net effect is a decreased ability to form clots, which is why warfarin is a vitamin K antagonist.

This differs from direct thrombin inhibitors (which would block thrombin itself), plasmin activators (which promote clot breakdown), or NSAIDs/antiplatelet drugs that inhibit COX-1 and platelet aggregation. Warfarin’s action is on the synthesis of vitamin K–dependent factors rather than on platelets or on plasmin-mediated fibrinolysis.