Which anticoagulant medication is a vitamin K antagonist?

Warfarin is classified as a vitamin K antagonist because it works by inhibiting the vitamin K epoxide reductase enzyme, which is essential for the synthesis of certain clotting factors in the liver. These clotting factors, including factors II, VII, IX, and X, require vitamin K for their activation. By antagonizing vitamin K, Warfarin decreases the levels of these clotting factors in circulation, thereby reducing the ability of blood to clot and serving as an effective anticoagulant for preventing thromboembolic events.

In contrast, heparin is an indirect acting anticoagulant that enhances the activity of antithrombin III, leading to the inactivation of thrombin and factor Xa, while dabigatran is a direct thrombin inhibitor. Rivaroxaban is a direct factor Xa inhibitor, both of which do not involve vitamin K in their mechanism of action. Thus, of the options listed, Warfarin is the only medication that functions as a vitamin K antagonist.