## **Core Concept**
The patient's presentation and lab results suggest a bleeding disorder related to a coagulation factor deficiency. The key findings include a normal platelet count, normal PT, elevated PTT, normal bleeding time, normal factor VIII, reduced factor IX, and a normal ristocetin cofactor assay. These results point towards a specific coagulation factor deficiency.

## **Why the Correct Answer is Right**
The patient's lab results show a normal PT and bleeding time but an elevated PTT, which indicates a problem in the intrinsic pathway of coagulation. The normal factor VIII level helps to rule out Hemophilia A, while the reduced level of factor IX points towards Hemophilia B, also known as Christmas disease. The normal ristocetin cofactor assay helps to confirm that the von Willebrand factor is functioning normally, which is often assessed with this test. Therefore, the diagnosis of Hemophilia B is the most likely.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option would be incorrect because it does not match the provided lab results and clinical scenario that points towards a factor IX deficiency.
- **Option B:** This would be incorrect because Hemophilia A involves a deficiency in factor VIII, which is normal in this patient.
- **Option D:** This option is incorrect because von Willebrand disease primarily affects the von Willebrand factor and factor VIII levels (due to its role in stabilizing factor VIII), and the ristocetin cofactor assay is abnormal in von Willebrand disease.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that patients with Hemophilia B (Christmas disease) have a deficiency in factor IX, leading to an elevated PTT and a bleeding tendency similar to that seen in Hemophilia A but with a different deficient factor. This condition is inherited in an X-linked recessive pattern, primarily affecting males.

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