**Question:** Which is true about Haemophilia A?

A. Haemophilia A is a genetic disorder characterized by a deficiency in factor VIII clotting factor.
B. Haemophilia A is caused by mutations in the factor IX gene, leading to reduced factor IX levels.
C. Haemophilia A is a bleeding disorder caused by a deficiency in factor VII clotting factor.
D. Haemophilia A is a disease caused by an increase in von Willebrand factor levels, leading to bleeding.

**Correct Answer:** Haemophilia A is a genetic disorder characterized by a deficiency in factor VIII clotting factor.

**Core Concept:** Haemophilia A is a bleeding disorder caused by a deficiency in one of the clotting factors, specifically factor VIII. The deficiency in factor VIII leads to impaired blood clotting, resulting in prolonged bleeding episodes when injured.

**Why the Correct Answer is Right:** Factor VIII is a critical component of the intrinsic pathway of the blood clotting cascade. It plays a vital role in converting factor IXa and X to their activated forms, which are essential for the formation of thrombin and ultimately, fibrin clot formation. The deficiency in factor VIII leads to a prothrombin time (PT) and activated partial thromboplastin time (aPTT) prolongation, which is a hallmark of haemophilia A.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because haemophilia A is caused by a deficiency in factor VIII, not factor IX. Haemophilia B (Christmas disease) is caused by a deficiency in factor IX.

B. This option is incorrect because mutations in the factor IX gene lead to reduced factor IX levels, not haemophilia A. Haemophilia A is caused by a deficiency in factor VIII.

C. This option is incorrect because haemophilia A is caused by a deficiency in factor VIII, not factor VII. Factor VII is not directly involved in the clotting cascade, unlike factor VIII.

D. This option is incorrect because an increase in von Willebrand factor levels would lead to bleeding disorders, not a decreased clotting function. Haemophilia A is caused by a deficiency in factor VIII.

**Clinical Pearl:** Haemophilia A is a severe bleeding disorder that primarily affects males. The deficiency in factor VIII leads to a prothrombin time (PT) and activated partial thromboplastin time (aPTT) prolongation, indicating impaired blood clotting. Early recognition and appropriate management of haemophilia A can prevent severe bleeding complications and improve the patient's quality of life.