In hemophilia A, the deficient factor is Factor VIII. This factor normally acts as a cofactor for which enzyme in the intrinsic coagulation pathway?
- A Factor IXa ✓
- B Factor Xa
- C Factor XIa
- D Thrombin
Correct answer: A. Factor IXa
Explanation
Factor VIII is the cofactor for Factor IXa in the intrinsic 'tenase' complex (Factor VIIIa + Factor IXa + calcium + phospholipid), which activates Factor X to Xa. Without Factor VIII, the intrinsic tenase complex cannot form efficiently, causing severely impaired thrombin generation and bleeding. Hemophilia A (Factor VIII deficiency) and B (Factor IX deficiency) both prolong aPTT with a normal PT.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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