Extrinsic pathway of coagulation is tested by?

**Core Concept**
The extrinsic pathway of coagulation is a critical component of the blood coagulation cascade, involving tissue factor and factor VIIa. This pathway is initiated by external trauma that leads to blood vessel damage. The extrinsic pathway converges with the intrinsic pathway to form the common coagulation pathway.

**Why the Correct Answer is Right**
The prothrombin time (PT) test is used to evaluate the extrinsic coagulation pathway. It measures the time taken for blood to clot after adding tissue factor, thus assessing the functionality of factors II, V, VII, X, and fibrinogen.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it tests a different aspect of coagulation.
**Option B:** Incorrect as it is not directly related to the extrinsic pathway.
**Option C:** Incorrect because it is not a standard test for coagulation pathways.

**Clinical Pearl / High-Yield Fact**
Remember that the prothrombin time (PT) and international normalized ratio (INR) are crucial for monitoring patients on warfarin therapy, as they reflect the extrinsic pathway's function.

**Correct Answer:** D. Prothrombin Time.