Which clotting factor is directly affected by vitamin K?

Vitamin K plays a crucial role in the synthesis of certain clotting factors within the liver. Specifically, it is essential for the post-translational modification of several clotting factors, helping them become active. Factor II, also known as prothrombin, is one such factor that is synthesized in a vitamin K-dependent manner.

Prothrombin is transformed into thrombin in the clotting cascade, a vital step in blood coagulation that converts fibrinogen into fibrin, thus forming a stable blood clot. The vitamin K-dependent factors also include Factors VII, IX, and X, but prothrombin is the primary factor that directly refers back to vitamin K in terms of clotting mechanism.

The other factors mentioned, such as Factor V, Factor VIII, and Factor XI, do not rely on vitamin K for their synthesis or activation; rather, their functions and activation pathways are distinct and do not involve vitamin K-dependent processes. Hence, prothrombin is the correct choice as it is the only clotting factor among those listed that is directly affected by vitamin K.