Hemophilia A is a deficiency of which clotting factor?

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

Answer: (A)

Factor VIII serves as a crucial cofactor for activated Factor IX in the intrinsic coagulation pathway, forming the tenase complex that activates Factor X to Xa. When Factor VIII is missing, this step is impaired, leading to reduced thrombin generation and poor clot formation. Clinically, that produces bleeding tendencies such as spontaneous or traumatic mucosal bleeding and joint hemorrhages, with a characteristic lab pattern of a prolonged activated partial thromboplastin time and a normal prothrombin time. Desmopressin can boost endogenous Factor VIII stores in milder cases, while more severe cases require replacement therapy with Factor VIII concentrate. The other factors listed correspond to different parts of the cascade (Factor IX deficiency causes a similar but separate condition, Factor VII is part of the extrinsic pathway, and Factor X is in the common pathway), so Factor VIII deficiency is what defines Hemophilia A.