Which laboratory test is MOST appropriate for evaluating a patient with suspected von Willebrand disease?

Correct answer: B. Platelet function assay / bleeding time

• A. INR assesses the EXTRINSIC pathway and is used to monitor warfarin. It is normal in von Willebrand disease. (The aPTT may be mildly prolonged when Factor VIII is reduced, but INR is not the test.)
• B. CORRECT. Von Willebrand factor performs two jobs: it mediates PLATELET ADHESION to injured endothelium, and it carries and stabilizes Factor VIII. The defect that manifests clinically is the PLATELET ADHESION failure, which is why screening rests on platelet function testing (the PFA-100 closure time, historically the bleeding time). Definitive diagnosis requires vWF antigen, ristocetin cofactor activity and Factor VIII level.
• C. Fibrinogen assesses the final common pathway and is unrelated to von Willebrand factor.
• D. A complete blood count reports the platelet NUMBER, which is normal in von Willebrand disease. The problem is platelet FUNCTION, not count — so the CBC is unrevealing.

Note: Von Willebrand disease is the MOST COMMON inherited bleeding disorder (roughly 1% of the population), usually autosomal dominant, and typically presents with mucocutaneous bleeding — epistaxis, easy bruising, menorrhagia and prolonged bleeding after extraction. Treatment: DESMOPRESSIN (DDAVP) for type 1, antifibrinolytics such as tranexamic acid, and vWF-containing concentrate for severe types. Avoid aspirin and NSAIDs.