A 10-year-old boy presents with spontaneous hemarthroses of the right knee. His maternal uncle had similar bleeding episodes. Bleeding time and PT are normal, but aPTT is prolonged. Mixing study with normal plasma corrects the aPTT. Factor VIII activity is normal, but factor IX activity is 2%. What is the inheritance pattern of this disorder?

• A Autosomal dominant
• B Autosomal recessive
• C X-linked recessive ✓
• D Mitochondrial

Explanation

Hemophilia B (factor IX deficiency) is X-linked recessive, identical in inheritance to hemophilia A. The normal bleeding time and PT with prolonged aPTT localizes the defect to the intrinsic pathway. Normal factor VIII with low factor IX confirms hemophilia B. The maternal uncle's history supports X-linked inheritance: the mother is a carrier who passed the affected X chromosome to her son.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.