Hemophilia B is due to:
**Core Concept**
Hemophilia B, also known as Christmas disease, is a rare X-linked recessive genetic disorder caused by a deficiency of the clotting factor IX. This deficiency leads to impaired blood coagulation and a predisposition to bleeding.
**Why the Correct Answer is Right**
The correct answer is due to a deficiency of factor IX, which is essential for the intrinsic pathway of blood coagulation. Factor IX is activated to factor IXa by factor XIa, and then it forms a complex with factor VIIIa to activate factor X. Without sufficient factor IX, the coagulation cascade is disrupted, leading to prolonged bleeding times.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not correct because Hemophilia B is not caused by a deficiency of factor VIII, which is the primary deficiency associated with Hemophilia A.
**Option B:** This option is not correct because Hemophilia B is not caused by a deficiency of von Willebrand factor, which is a different type of bleeding disorder.
**Option C:** This option is not correct because Hemophilia B is not caused by a deficiency of factor XIII, which is involved in the stabilization of blood clots.
**Clinical Pearl / High-Yield Fact**
The severity of Hemophilia B is often correlated with the degree of factor IX deficiency, with patients having <1% factor IX activity typically experiencing severe bleeding, while those with 1-5% activity may experience mild bleeding.
**Correct Answer:** C. Deficiency of factor IX.