Hemophilia B is due to decreased activity of which factor?

Hemophilia B is due to decreased activity of which factor?

• A. Factor IX
• B. Factor VIII
• C. Factor X
• D. Factor II

Answer: (A)

Hemophilia B is caused by decreased activity of factor IX, the Christmas factor. Factor IX is a vitamin K–dependent protein of the intrinsic coagulation pathway. In this pathway, factor IXa, with factor VIIIa on a phospholipid surface, activates factor X to Xa, driving thrombin generation and clot formation. When factor IX activity is reduced, this amplification is impaired, leading to less thrombin and a tendency to bleeding, especially into joints and muscles. Clinically, the aPTT is typically prolonged while the PT can be normal. The disorder is inherited in an X‑linked pattern. This distinguishes it from Hemophilia A, which is due to factor VIII deficiency, and from defects in the common pathway factors like X or II that affect the cascade more broadly.

Hemophilia B is caused by decreased activity of factor IX, the Christmas factor. Factor IX is a vitamin K–dependent protein of the intrinsic coagulation pathway. In this pathway, factor IXa, with factor VIIIa on a phospholipid surface, activates factor X to Xa, driving thrombin generation and clot formation. When factor IX activity is reduced, this amplification is impaired, leading to less thrombin and a tendency to bleeding, especially into joints and muscles. Clinically, the aPTT is typically prolonged while the PT can be normal. The disorder is inherited in an X‑linked pattern. This distinguishes it from Hemophilia A, which is due to factor VIII deficiency, and from defects in the common pathway factors like X or II that affect the cascade more broadly.