In vitro coagulation is initiated by factor:
**Core Concept**
The coagulation cascade is a complex process that involves the sequential activation of various clotting factors, ultimately leading to the formation of a fibrin clot. In vitro coagulation refers to the process of blood clotting that occurs outside the body, often in a laboratory setting.
**Why the Correct Answer is Right**
In vitro coagulation is initiated by the exposure of blood to a negatively charged surface, which activates the contact activation pathway. The primary initiator of this pathway is factor XII (Hageman factor), which is a serine protease that plays a crucial role in the activation of the coagulation cascade. When blood comes into contact with a negatively charged surface, factor XII is activated, leading to the formation of factor XIa, which then activates factor IX, and subsequently, the coagulation cascade is initiated.
**Why Each Wrong Option is Incorrect**
* **Option A:** Factor VII is a key component of the extrinsic pathway of coagulation, which is initiated by tissue damage. However, it is not the primary initiator of in vitro coagulation.
* **Option B:** Factor VIII is a critical component of the intrinsic pathway of coagulation, but it is not the primary initiator of in vitro coagulation.
* **Option D:** Factor XIII is a transglutaminase that stabilizes the fibrin clot, but it is not the primary initiator of in vitro coagulation.
**Clinical Pearl / High-Yield Fact**
The contact activation pathway is highly sensitive to heparin, which is why patients on heparin therapy require close monitoring to avoid bleeding complications.
**Correct Answer: C. Factor XII.