**Question:** Activated protein C inhibit clotting mechanism by inactivating which of the following clotting factors:

A. Factors Va and VIIIa
B. Factor Xa
C. Factor Xa and Factor IXa
D. Factor Xa, IXa, and XIa

**Core Concept:**
Activated protein C is an important component of the physiological anticoagulation system, primarily involving the regulation of coagulation by inactivating specific clotting factors. In this scenario, we are assessing the clotting factors that are targeted by activated protein C.

**Why the Correct Answer is Right:**
Activated protein C primarily inhibits clotting through the inactivation of Factor Xa and Factor V. This complex process is known as the intrinsic pathway of coagulation, involving the interaction of activated protein C with Factor Va and VIIIa. This interaction results in the proteolytic inactivation of Factor Xa and Factor V, leading to a reduction in clot formation.

**Why Each Wrong Option is Incorrect:**
Option A (Factors Va and VIIIa) is partially correct as these factors do interact with activated protein C, but they are not solely responsible for the clotting inhibition mediated by activated protein C. Inactivating Factor Xa and Factor V is crucial for the anticoagulant effect of activated protein C.

Option B (Factor Xa) is partially correct as Factor Xa is indeed inactivated by activated protein C, but its inclusion does not account for the complete inactivation of clotting factors by activated protein C.

Option C (Factor Xa and Factor IXa) is incorrect because Factor IXa is not involved in the anticoagulant effect of activated protein C. The primary target clotting factors are Factor Xa and Factor V.

Option D (Factor Xa, IXa, and XIa) is incorrect because Factor XIa is not directly involved in the anticoagulant effect of activated protein C. The primary targets of activated protein C are Factor Xa and Factor V, which play a crucial role in clotting cascade activation.

**Clinical Pearl:**
Understanding the anticoagulant effect of activated protein C is essential for clinicians to effectively manage patients with coagulation disorders or those undergoing anticoagulant therapy. This knowledge helps in choosing the appropriate treatment strategies and monitoring their efficacy.

**Correct Answer:**

Correct Answer: B. Factor Xa

**Why Factor Xa is Right:**
Activated protein C directly interacts with activated Factor Xa, leading to its inactivation. This inactivation of Factor Xa is a vital part of the anticoagulant effect of activated protein C, contributing to the regulation of coagulation and preventing excessive blood clot formation.