**Question:** Hemophilia B is due to deficiency of:

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

**Correct Answer:** B. Factor IX

**Core Concept:** Hemophilia B is a bleeding disorder caused by deficiency of clotting factor proteins, which help blood clotting to occur properly. There are several clotting factors involved in the process, and each factor has a specific role in the coagulation cascade.

**Why the Correct Answer is Right:** Hemophilia B is caused by deficiency of Factor IX, also known as Christmas disease. Factor IX plays a crucial role in the intrinsic pathway of blood coagulation. When Factor IX is deficient or inactive, the intrinsic pathway is disrupted, leading to prolonged bleeding times and increased susceptibility to bleeding episodes.

**Why Each Wrong Option is Incorrect:**

A. Factor VIII deficiency (Hemophilia A) is responsible for the majority of cases of severe hemophilia, not Hemophilia B. Hemophilia A is characterized by deficiency of Factor VIII, which is involved in the common pathway of blood clotting.

B. Factor X deficiency (Hemophilia C) is responsible for a small percentage of cases of severe hemophilia, different from Hemophilia B. Hemophilia C is characterized by deficiency of Factor X, which is involved in the common pathway of blood clotting.

C. Factor V deficiency is not associated with any bleeding disorder, making it an incorrect answer choice for Hemophilia B. Factor V plays a role in the common pathway of blood clotting, but its deficiency does not cause bleeding disorders.

**Clinical Pearl:** Hemophilia B is a rare genetic disorder affecting the clotting cascade, and the deficiency of Factor IX leads to an increased risk of spontaneous or prolonged bleeding episodes. Early diagnosis and appropriate management can significantly improve the quality of life for individuals affected by Hemophilia B.