A 5-year-old boy presents with hemarthroses and prolonged aPTT. PT and bleeding time are normal. Factor VIII activity is 1%. Which inheritance pattern applies and what distinguishes this from hemophilia B?

• A Hemophilia A — X-linked recessive, factor VIII deficiency; hemophilia B is X-linked recessive, factor IX deficiency ✓
• B Hemophilia A — autosomal recessive, factor VIII deficiency; hemophilia B is autosomal dominant factor IX deficiency
• C Hemophilia A — X-linked dominant, factor VIII deficiency; hemophilia B is X-linked recessive
• D Hemophilia A and B are clinically indistinguishable except by factor XII mixing studies

Explanation

Both hemophilia A (factor VIII deficiency) and hemophilia B (factor IX deficiency, Christmas disease) are X-linked recessive disorders caused by mutations in different genes on the X chromosome. Both present identically — hemarthroses, deep muscle hematomas, and prolonged aPTT with normal PT. They are distinguished only by specific factor assays: factor VIII is reduced in hemophilia A and normal in hemophilia B (and vice versa). Clinically and by coagulation screening they are indistinguishable, requiring specific factor activity testing.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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