A 6-year-old boy presents with recurrent hemarthroses of the knees. aPTT is prolonged, PT is normal, platelet count and bleeding time are normal. Mixing studies show the prolonged aPTT corrects with normal plasma and with factor VIII-deficient plasma, but NOT with factor IX-deficient plasma. Which factor is deficient?

• A Factor IX ✓
• B Factor VIII
• C Factor XI
• D von Willebrand factor

Explanation

Correction with factor VIII-deficient plasma (which still contains factor IX) but not with factor IX-deficient plasma confirms factor IX deficiency (hemophilia A). If the patient had factor VIII deficiency, mixing with factor VIII-deficient plasma would not correct. Both hemophilia B and A are X-linked and present with prolonged aPTT and hemarthroses.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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