## **Core Concept**
Hemophilia is a genetic disorder characterized by the deficiency of one of the clotting factors, specifically factor VIII (Hemophilia A) or factor IX (Hemophilia B). This deficiency leads to impaired blood coagulation, manifesting as prolonged bleeding. Laboratory diagnosis often involves assessing various coagulation parameters.

## **Why the Correct Answer is Right**
The correct answer, **PTT (Partial Thromboplastin Time)**, is raised in hemophilia because PTT measures the integrity of the intrinsic and common coagulation pathways, which involve factors VIII and IX. Since hemophilia A and B result from deficiencies of these factors, the PTT, which evaluates the efficiency of these pathways, is prolonged.

## **Why Each Wrong Option is Incorrect**
- **Option A: Platelet count** is not directly related to the coagulation defects seen in hemophilia. Hemophilia primarily affects the coagulation cascade, not platelet count or function.
- **Option B: PT (Prothrombin Time)** measures the extrinsic and common coagulation pathways. Since factors VIII and IX are not part of the extrinsic pathway, PT is typically normal in hemophilia.
- **Option D: Bleeding time** assesses platelet function and primary hemostasis. Hemophilia affects secondary hemostasis (coagulation), so bleeding time is usually normal.

## **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 deficiencies (factor VIII for Hemophilia A and factor IX for Hemophilia B) and by the laboratory finding of a prolonged PTT with a normal PT.

## **Correct Answer:** . PTT.