## **Core Concept**
The question tests understanding of bleeding disorders, specifically those related to deficiencies in clotting factors. The key concept here involves distinguishing between different types of hemophilia, which are characterized by deficiencies in specific clotting factors. Hemophilia A is caused by a deficiency in **factor VIII**, while Hemophilia B is caused by a deficiency in **factor IX**.

## **Why the Correct Answer is Right**
The correct answer, **Hemophilia B**, is appropriate because the boy has reduced serum levels of **factor IX** but normal levels of **factor VIII**. Hemophilia B, also known as Christmas disease, is a genetic disorder caused by missing or low levels of the **factor IX** clotting protein. This condition leads to problems with blood clotting, resulting in prolonged bleeding. The presentation of severe bleeding into a joint, such as the knee, is a common complication of hemophilia B.

## **Why Each Wrong Option is Incorrect**
- **Option A (Hemophilia A):** This is incorrect because Hemophilia A involves a deficiency in **factor VIII**, but the question states that levels of factor VIII are normal.
- **Option C (von Willebrand disease):** This is incorrect because von Willebrand disease is related to a deficiency or dysfunction of **von Willebrand factor**, which is involved in the adhesion of platelets to damaged blood vessels and serves as a carrier protein for factor VIII. The question focuses on factors VIII and IX.
- **Option D (Disseminated intravascular coagulation):** This is incorrect because disseminated intravascular coagulation (DIC) is a condition characterized by both widespread clotting and bleeding in the vascular system, not a specific deficiency of a single clotting factor like factor IX.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Hemophilia A and B are X-linked recessive disorders, primarily affecting males, while females are usually carriers. A classic clinical presentation, as seen in this case, involves spontaneous bleeding into joints (hemarthrosis), which can lead to chronic joint disease if not properly managed.

## **Correct Answer:** . **Hemophilia B**