Hemophilia B is due to deficiency of
**Core Concept**
Hemophilia B, also known as Christmas disease, is a genetic disorder caused by a deficiency of a specific clotting factor. This deficiency leads to impaired blood coagulation, resulting in prolonged bleeding after injuries or spontaneous bleeding episodes.
**Why the Correct Answer is Right**
Hemophilia B is caused by a deficiency of factor IX, a clotting factor synthesized in the liver and stored in platelets. Factor IX plays a critical role in the intrinsic pathway of blood coagulation, where it forms a complex with factor VIII and calcium ions to activate factor X. Without sufficient factor IX, the coagulation cascade is disrupted, leading to impaired clot formation.
**Why Each Wrong Option is Incorrect**
**Option A:** Factor VIII - Incorrect because Hemophilia B is caused by a deficiency of factor IX, not factor VIII. Factor VIII deficiency leads to Hemophilia A.
**Option B:** Factor VII - Incorrect because Factor VII is involved in the extrinsic pathway of blood coagulation, not the intrinsic pathway affected in Hemophilia B.
**Option C:** Factor XI - Incorrect because Factor XI is involved in the intrinsic pathway, but its deficiency leads to a different bleeding disorder, not Hemophilia B.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that Hemophilia A and B have distinct clinical presentations and laboratory findings. Hemophilia A is more common and often presents with bleeding episodes after minor trauma, while Hemophilia B typically presents with more severe bleeding episodes, particularly in joints and muscles.
**Correct Answer: C. Factor IX**