**Core Concept**
Hemophilia is a genetic disorder characterized by the deficiency or dysfunction of clotting factors, leading to spontaneous or traumatic bleeding. The primary clinical manifestations of hemophilia are related to the accumulation of blood in joints and muscles due to impaired coagulation.

**Why the Correct Answer is Right**
Hemarthrosis is a hallmark of hemophilia due to the accumulation of blood in the joint space, typically occurring in weight-bearing joints such as the knees, elbows, and ankles. This is a result of the impaired coagulation cascade, leading to the inability to form a stable blood clot. The repeated episodes of hemarthrosis can cause joint damage, pain, and deformity. In hemophilia A, the deficiency of factor VIII leads to impaired intrinsic pathway coagulation, whereas in hemophilia B, the deficiency of factor IX affects the intrinsic and common coagulation pathways.

**Why Each Wrong Option is Incorrect**
**Option A:** Epistaxis is a common symptom of various bleeding disorders, but it is not specific to hemophilia. Patients with hemophilia may experience epistaxis, but it is not a hallmark of the condition.
**Option C:** Abdominal pain can occur in hemophilia due to gastrointestinal bleeding or splenic rupture, but it is not a characteristic feature of the disease. Hemarthrosis is a more specific and common manifestation.
**Option D:** Anaemia is a potential complication of hemophilia due to chronic blood loss, but it is not a hallmark of the disease. Hemarthrosis is the primary clinical manifestation of hemophilia.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that hemarthrosis can occur in any joint, but weight-bearing joints are more commonly affected. A thorough physical examination and imaging studies (e.g., X-rays, ultrasound) are crucial in diagnosing hemarthrosis in patients with suspected hemophilia.

**✓ Correct Answer: B. Hemarthrosis**