In Hemophilia A, which factor is deficient?

• A. Factor IX
• B. Factor XI
• C. Factor VIII
• D. Factor VII

Answer: (C)

Factor VIII deficiency defines Hemophilia A. Factor VIII acts as a cofactor for activated Factor IX (IXa) in the tenase complex, which is essential for converting Factor X to Xa and generating thrombin. Without enough Factor VIII, the intrinsic pathway cannot effectively drive clot formation, leading to easy and recurrent bleeding, especially into joints and muscles, and a prolonged aPTT with a normal PT. This distinguishes it from other bleeding disorders: a deficiency of Factor IX causes Hemophilia B, Factor XI deficiency causes Hemophilia C, and a deficiency of Factor VII affects the extrinsic pathway and PT. Treatments include desmopressin to release stored Factor VIII in milder cases and Factor VIII replacement for more severe disease.