**Question:** Clotting factor common to extrinsic and intrinsic pathway is:

A. Factor VII
B. Factor IX
C. Factor X
D. Factor VIII

**Core Concept:**
The coagulation cascade, also known as the clotting or haemostasis system, is a complex series of reactions that result in the formation of a blood clot. This process is essential to prevent excessive bleeding and maintain haemostasis. The extrinsic and intrinsic pathways are two main pathways that converge to activate factors leading to clot formation.

**Why the Correct Answer is Right:**
The correct answer, Factor X, is a key clotting factor involved in both the extrinsic and intrinsic pathways of the coagulation cascade. In the intrinsic pathway, Factor X is activated by Factor VIIa and tissue factor complex. In the extrinsic pathway, Factor X is activated by the prothrombinase complex, which includes activated Factor XI, Factor VIIa, and Factor V. Both pathways converge at the activation of Factor X, which then proceeds to form the common pathway enzymes, Factor Xa and Factor IIa (thrombin).

**Why Each Wrong Option is Incorrect:**

A. Factor VII (Option A) is involved in the extrinsic pathway, where it is activated by tissue factor complex. However, it is not a part of the intrinsic pathway and thus is incorrect.
B. Factor IX (Option B) is primarily involved in the intrinsic pathway, where it is activated by Factor VIII. Although Factor IX plays a role in both pathways, it is not a common factor for both.
C. Factor X (Option C) is the correct answer, as explained above.
D. Factor VIII (Option D) is primarily involved in the intrinsic pathway, where it interacts with Factor IXa to activate Factor X. However, it is not a common factor for both pathways.

**Clinical Pearl:**
Understanding the coagulation cascade and the roles of various clotting factors is crucial for medical professionals, as it has implications in diagnosing and treating individuals with bleeding disorders. Knowledge of these pathways also aids in interpreting laboratory test results and deciding appropriate treatment strategies, such as administering Factor VIII or Von Willebrand factor for bleeding disorders.