A 12-year-old boy has recurrent hemarthroses of the knee. PT, platelet count and bleeding time are normal. aPTT is prolonged and corrects completely on mixing with normal plasma. Factor VIII activity is normal. Which single factor assay will establish the diagnosis?

• A Factor XI assay
• B Factor VII assay
• C Factor IX assay ✓
• D Factor X assay

Explanation

Hemophilia C (Christmas disease) is an X-linked recessive deficiency of factor IX, presenting like hemophilia A with isolated prolonged aPTT, normal PT and normal platelet-dependent tests. Complete correction on mixing confirms a factor deficiency rather than an inhibitor, and a normal factor VIII level excludes hemophilia A, making factor IX assay diagnostic. Factor VII deficiency prolongs PT, not aPTT, while factor X deficiency prolongs both.

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.