Which of the following is not involved in intrinsic pathway
**Core Concept**
The intrinsic pathway of blood coagulation involves a series of enzyme-catalyzed reactions that ultimately lead to the formation of thrombin and fibrin from fibrinogen. This pathway is primarily responsible for the regulation of coagulation within the vascular system.
**Why the Correct Answer is Right**
The intrinsic pathway is initiated by the binding of factor XII (Hageman factor) to negatively charged surfaces, such as collagen or glass. This binding leads to the activation of factor XI, which in turn activates factor IX. The activated factor IX then binds to factor VIIIa, forming a complex that activates factor X. The intrinsic pathway is essential for the regulation of coagulation in response to vascular injury.
**Why Each Wrong Option is Incorrect**
**Option A:** Factor VII is not involved in the intrinsic pathway, but rather in the extrinsic pathway, which is activated by tissue factor.
**Option B:** Factor X is indeed involved in the intrinsic pathway, as it is activated by the factor IX-factor VIIIa complex.
**Option C:** Factor XIII is involved in the stabilization of fibrin clots, but it is not a part of the intrinsic pathway.
**Option D:** Factor VIII is a crucial component of the intrinsic pathway, as it forms a complex with activated factor IX to activate factor X.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the intrinsic pathway is activated by contact with negatively charged surfaces, whereas the extrinsic pathway is activated by tissue factor.
**Correct Answer: A. Factor VII is not involved in the intrinsic pathway.**