Which of the following coagulation factors is deficient in classical haemophilia:
**Core Concept**
Classical haemophilia, also known as haemophilia A, is a genetic disorder characterized by a deficiency of a specific coagulation factor, leading to prolonged bleeding and spontaneous joint hemorrhages. This disorder is inherited in an X-linked recessive pattern, predominantly affecting males.
**Why the Correct Answer is Right**
The coagulation cascade is a complex process involving multiple enzymes and proteins. Classical haemophilia is caused by a deficiency of factor VIII, a crucial component of the intrinsic pathway. Factor VIII acts as a cofactor for factor IXa in the activation of factor X. Without sufficient factor VIII, the intrinsic pathway is impaired, resulting in an inability to form a stable blood clot. The deficiency of factor VIII can be due to genetic mutations, such as deletions, insertions, or point mutations, which disrupt the normal function of the gene.
**Why Each Wrong Option is Incorrect**
**Option A:** Factor II is involved in the extrinsic pathway of coagulation, not the intrinsic pathway affected in classical haemophilia.
**Option B:** Factor XI is a component of the intrinsic pathway but is not the primary factor deficient in classical haemophilia.
**Option C:** Factor XIII is involved in the stabilization of blood clots, not the initiation of coagulation, making it an unlikely candidate for classical haemophilia.
**Clinical Pearl / High-Yield Fact**
Classic symptoms of classical haemophilia include easy bruising, spontaneous joint hemorrhages, and prolonged bleeding after minor trauma. Patients with classical haemophilia may also experience hemarthrosis, which can lead to chronic joint inflammation and degenerative joint disease.
**Correct Answer:** C. Factor VIII.