Which vitamin's activity is affected by Warfarin, leading to anticoagulation?

Warfarin is an anticoagulant medication that functions by inhibiting vitamin K-dependent clotting factors. This means that it interferes with the synthesis of clotting factors II, VII, IX, and X, which are critical for normal blood coagulation. Vitamin K is essential for the post-translational modification of these clotting factors, allowing them to bind calcium and function properly in the coagulation cascade. When Warfarin is administered, it effectively reduces the availability of active vitamin K, thereby diminishing the production of functional clotting factors and leading to anticoagulation.

The other vitamins listed do not have a similar direct role in the coagulation process influenced by Warfarin. Vitamin A, D, and E are important for various physiological functions, such as vision, bone health, and antioxidant activity, respectively, but they do not play a critical role in the synthesis of clotting factors affected by Warfarin. Therefore, vitamin K is the correct choice as it is directly involved in the mechanism of action of Warfarin.