The extrinsic pathway of clotting is activated by
**Core Concept**
The extrinsic pathway of blood coagulation is one of the two main pathways that lead to the formation of a blood clot. It is initiated outside the vascular system and involves the activation of tissue factor (TF) and factor VIIa (FVIIa).
**Why the Correct Answer is Right**
The extrinsic pathway is activated when there is trauma or injury to the tissues outside the blood vessels, leading to the release of tissue factor (TF). TF then binds to and activates factor VIIa (FVIIa), which in turn activates factor X (FX) and factor IX (FIX) through the intrinsic pathway, leading to the formation of thrombin and fibrin. This pathway is known as the "extrinsic" pathway because it is initiated outside the blood vessels.
**Why Each Wrong Option is Incorrect**
**Option A:** Factor XII (Hageman factor) is involved in the initiation of the intrinsic pathway of blood coagulation, not the extrinsic pathway.
**Option B:** Factor VIII (antihemophilic factor) is involved in the intrinsic pathway of blood coagulation and is a critical component of the hemostatic process.
**Option C:** Factor XIII (fibrin stabilizing factor) is involved in the stabilization of clots, not the initiation of the extrinsic pathway.
**Clinical Pearl / High-Yield Fact**
Factor VIIa (FVIIa) is a crucial component of the extrinsic pathway, and its deficiency can lead to a bleeding disorder known as hemophilia A, which is characterized by prolonged bleeding and clotting times.
**Correct Answer: D. Tissue factor (TF) and Factor VIIa (FVIIa).**