## **Core Concept**
The question tests understanding of coagulation disorders, specifically those that affect the intrinsic pathway of blood coagulation, which is evaluated by the activated partial thromboplastin time (APTT). The normal platelet count and PT (prothrombin time) suggest the disorder is not related to platelets or the extrinsic coagulation pathway.

## **Why the Correct Answer is Right**
The correct answer, **Hemophilia A**, is a genetic disorder caused by a deficiency in **Factor VIII**, a crucial component of the intrinsic coagulation pathway. This deficiency leads to a prolongation of the APTT, which measures the integrity of the intrinsic pathway, while PT remains normal as it assesses the extrinsic pathway. Hemophilia A primarily affects males due to its X-linked recessive inheritance pattern, but females can be carriers or, less commonly, express the disorder if they are homozygous for the mutation. The clinical presentation of a bleeding diathesis supports this diagnosis.

## **Why Each Wrong Option is Incorrect**
- **Option A (Hemophilia B):** This is incorrect because Hemophilia B is caused by a deficiency in **Factor IX**, not Factor VIII. Although it also presents with a prolonged APTT and a bleeding diathesis, the specific factor deficiency differs.
- **Option C (Von Willebrand Disease):** This is incorrect because, while Von Willebrand Disease (VWD) can present with a prolonged APTT and is a common cause of bleeding disorders, it primarily involves a deficiency or dysfunction of **Von Willebrand Factor (VWF)**, which is involved in platelet adhesion and also serves as a carrier protein for Factor VIII. However, the isolated prolongation of APTT without other suggestive features (like thrombocytopenia or specific types of VWD) makes it less likely here.
- **Option D (Disseminated Intravascular Coagulation):** This is incorrect because Disseminated Intravascular Coagulation (DIC) typically presents with both prolonged PT and APTT, along with thrombocytopenia, due to the consumption of clotting factors and platelets.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Hemophilia A and B can be distinguished from other bleeding disorders by their specific factor deficiencies and the pattern of inheritance. A significant clue to the diagnosis is the isolated prolongation of APTT with normal PT and platelet count, suggesting an intrinsic pathway defect.

## **Correct Answer:** . **Hemophilia A**