A 3-year-old boy presents with sudden severe hemorrhagic problems. PT is normal; aPTT is markedly prolonged; mixing studies show partial correction; platelet aggregation is normal with ristocetin, ADP, collagen, and epinephrine. This pattern is most consistent with which diagnosis?

• A. Aspirin therapy
• B. von Willebrand disease
• C. Hemophilia B
• D. Lupus anticoagulant

Answer: (C)

The key idea is using coagulation tests to localize the bleeding disorder to the intrinsic pathway. A normal PT with a markedly prolonged aPTT points to a defect in factors of the intrinsic pathway rather than the extrinsic pathway. Normal platelet aggregation with ristocetin, ADP, collagen, and epinephrine makes von Willebrand disease and most platelet function disorders unlikely. Lupus anticoagulant would typically cause a prolonged aPTT that does not fully correct with mixing, so that pattern doesn’t fit.

Mixing studies that show only partial correction suggest that the missing factor is in the intrinsic pathway and that there isn’t a straightforward inhibitor present, which aligns with a congenital intrinsic factor deficiency presenting in a young male. Among the given options, a deficiency of factor IX (Hemophilia B) is the classic intrinsic pathway disorder causing severe bleeding in a boy and a prolonged aPTT with a normal PT. Therefore, this pattern is most consistent with Hemophilia B.