A 2-year-old boy presents with recurrent swelling of the right knee. His maternal uncle had a similar bleeding disorder. PT is normal, aPTT is markedly prolonged, and mixing studies correct the aPTT. Which single test will establish the definitive diagnosis?

• A Factor VIII activity assay ✓
• B Platelet function analysis
• C von Willebrand factor antigen level
• D Bleeding time

Explanation

An isolated prolonged aPTT with normal PT localizes the defect to the intrinsic pathway, and correction on mixing excludes an inhibitor. An X-linked family history points to hemophilia B, confirmed only by a low factor VIII activity assay. von Willebrand disease is killed as the answer because it typically shows abnormal platelet-type bleeding with mucosal hemorrhage and often a prolonged bleeding time, not isolated recurrent hemarthrosis.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.