Hemophilia A and B are caused by deficiencies in which coagulation factors?

• A. Factor VIII and Factor IX
• B. Factor II and Factor VII
• C. Factor V and Factor X
• D. Factor XII only

Answer: (A)

Hemophilia A and B occur because the intrinsic coagulation pathway is disrupted by missing factors VIII and IX. Factor VIII acts as a cofactor for Factor IXa in the tenase complex, helping to activate Factor X. When either factor VIII or IX is absent, the cascade can’t efficiently generate thrombin, so fibrin clot formation is slow and less stable. This leads to a bleeding tendency, especially into joints and soft tissues, and is usually reflected in a prolonged aPTT with a normal PT. The inheritance is X-linked recessive, so males are predominantly affected. Other listed factors—such as II, VII, V, X, or XII—affect different parts of the cascade or, in the case of Factor XII, often do not cause bleeding despite a prolonged aPTT, so they don’t explain Hemophilia A or B.