Bernard-Soulier syndrome is characterized by which lab finding?

• A. Deficiency of GPIb-IX-V with abnormal ristocetin-induced aggregation and large platelets
• B. Normal ristocetin aggregation
• C. Deficiency of GPIIb-IIIa
• D. Low von Willebrand factor

Answer: (A)

Bernard-Soulier syndrome is a congenital platelet adhesion defect caused by deficiency of the GPIb-IX-V receptor on the platelet surface, which is the binding site for von Willebrand factor. Without this receptor, platelets cannot properly interact with vWF, so ristocetin-induced platelet aggregation is markedly abnormal. The condition also features thrombocytopenia with giant (large) platelets on the smear. Putting these clues together, a laboratory finding showing a deficiency of GPIb-IX-V with abnormal ristocetin-induced aggregation and large platelets best matches Bernard-Soulier syndrome.