Vitamin K dependent clotting factor is:
**Core Concept**
Vitamin K dependent clotting factors are a group of proteins synthesized in the liver that play a crucial role in the coagulation cascade. These proteins are essential for the formation of a stable blood clot, and their deficiency can lead to coagulopathy.
**Why the Correct Answer is Right**
Vitamin K dependent clotting factors include Factor II (prothrombin), Factor VII, Factor IX, and Factor X. These factors require vitamin K as a cofactor for the post-translational modification of glutamate residues to gamma-carboxyglutamate, which enables them to bind calcium ions and phospholipids, ultimately facilitating blood clot formation. The gamma-carboxylation reaction is catalyzed by the enzyme gamma-glutamyl carboxylase.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not a vitamin K dependent clotting factor. Factor III (tissue factor) is a cell surface receptor that initiates the coagulation cascade, but it is not dependent on vitamin K for its activity.
* **Option B:** This option is not a vitamin K dependent clotting factor. Factor VIII is an essential component of the intrinsic coagulation pathway, but it is synthesized in the liver and does not require vitamin K for its activity.
* **Option C:** This option is not a vitamin K dependent clotting factor. Factor XI is a plasma serine protease that participates in the intrinsic coagulation pathway, but it is not dependent on vitamin K for its activity.
**Clinical Pearl / High-Yield Fact**
Vitamin K dependent clotting factors are essential for the formation of a stable blood clot. Deficiency in these factors can lead to bleeding disorders, such as hemophilia A and B, which are characterized by a deficiency in Factors VIII and IX, respectively.
**Correct Answer: C. Factor X.**