**Core Concept:** Vitamin K is an essential cofactor that plays a crucial role in the post-translational modification of proteins involved in blood clotting through a process called carboxylation. These proteins are called clotting factors and are crucial to the normal functioning of the coagulation cascade.

**Why the Correct Answer is Right:** Vitamin K is necessary for the synthesis of certain clotting factors, particularly those involved in the extrinsic pathway of blood clotting. One of these clotting factors is Factor VII, also known as Prothrombin. Factor VII is activated through a series of enzymatic reactions resulting in its carboxylation, which is dependent on vitamin K. Once activated, Factor VII helps convert Factor X to its active form, Factor Xa, thereby facilitating the formation of thrombin and ultimately leading to the clotting of blood.

**Why Each Wrong Option is Incorrect:**

A. Factor II (Prothrombin) is another clotting factor, but unlike Factor VII, it does not require vitamin K for its synthesis.
B. Factor VIII is a clotting factor, but it is not vitamin K dependent.
C. Factor IX is a clotting factor, but it is not vitamin K dependent.
D. Factor X, like Factor VII, is activated through a series of enzymatic reactions, but its synthesis is not dependent on vitamin K.

**Clinical Pearl:** Vitamin K deficiency or antagonists can lead to prolonged activated partial thromboplastin time (aPTT), a test used to assess the coagulation cascade, which is indicative of impaired blood clotting.

**Correct Answer:** D. Factor X is a vitamin K-dependent clotting factor, as its synthesis is dependent on vitamin K, similar to Factor VII.