**Core Concept**
Christmas disease, also known as hemophilia B, is a genetic disorder characterized by a deficiency of a specific clotting factor in the blood. This leads to prolonged bleeding and an increased risk of bleeding complications. The underlying principle is the importance of clotting factors in the coagulation cascade.

**Why the Correct Answer is Right**
Christmas disease is caused by a deficiency of factor IX, a vitamin K-dependent clotting factor that plays a crucial role in the intrinsic pathway of blood coagulation. Factor IX is synthesized in the liver and circulates in the blood, where it forms a complex with factor VIII to activate factor X. A deficiency of factor IX leads to impaired clot formation, resulting in prolonged bleeding. The deficiency is usually inherited in an X-linked recessive pattern, affecting males more frequently than females.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because Christmas disease is not caused by a deficiency of factor VIII, which is the cause of hemophilia A.
**Option B:** This is incorrect because von Willebrand disease is caused by a deficiency of von Willebrand factor, not factor IX.
**Option C:** This is incorrect because factor XI deficiency is a different bleeding disorder known as hemophilia C.

**Clinical Pearl / High-Yield Fact**
Christmas disease is an X-linked recessive disorder, which means that affected males are at risk of passing the mutated gene to their daughters, who can become carriers. Female carriers of the mutated gene have a 50% chance of passing it to each of their sons, who will be affected, and a 50% chance of passing it to each of their daughters, who will be carriers.

**Correct Answer:** D. Factor IX.