**Core Concept**
Hemophilia B, also known as Christmas disease, is a genetic disorder caused by a deficiency of a specific clotting factor in the blood. This leads to impaired blood coagulation and increased bleeding tendency.

**Why the Correct Answer is Right**
Hemophilia B 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 is necessary for the conversion of factor X to factor Xa, which ultimately leads to the formation of a stable blood clot. A deficiency of factor IX results in impaired blood clotting, leading to excessive bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Hemophilia A is caused by a deficiency of factor VIII, not factor IX.

**Option B:** This option is incorrect because factor XI deficiency is known as hemophilia C, which is a separate bleeding disorder.

**Option C:** This option is incorrect because factor XIII is a transglutaminase that stabilizes blood clots, but its deficiency does not cause hemophilia.

**Clinical Pearl / High-Yield Fact**
The most common symptoms of Hemophilia B include joint pain and swelling, easy bruising, and prolonged bleeding after minor injuries. Factor IX deficiency can be diagnosed with a simple blood test, and treatment involves replacement therapy with recombinant factor IX.

**Correct Answer:** B. Factor IX deficiency is the hallmark of Hemophilia B.