USMLE Step 3 Drug MOA Practice Test 2025 - Free Drug Mechanism of Action Practice Questions and Study Guide

Warfarin exerts its anticoagulant effect primarily by inhibiting vitamin K epoxide reductase, an enzyme critical in the vitamin K cycle. This enzyme is responsible for converting vitamin K epoxide back to its active form, vitamin K hydroquinone. Vitamin K is essential for the post-translational modification of several clotting factors, specifically factors II (prothrombin), VII, IX, and X, as well as proteins C and S, which are vital in the coagulation cascade.

By inhibiting this enzyme, warfarin effectively reduces the levels of active vitamin K, leading to decreased synthesis of these vitamin K-dependent clotting factors. As a result, the anticoagulant effect is achieved because the reduced levels of these clotting factors impair the blood's ability to coagulate, helping to prevent thrombus formation.

The other options relate to different mechanisms that do not directly pertain to the action of warfarin. For instance, inhibiting the synthesis of thromboxane A2 would relate more to agents like aspirin that impact platelet function. Decreased sodium absorption in the kidneys and inhibiting calcium influx into platelets are irrelevant to the mechanism of action of warfarin and pertain to other therapeutic areas. Understanding