A 4-year-old boy has lifelong recurrent epistaxis and gum bleeding. Investigations: platelet count 70,000/uL with giant platelets on smear, prolonged bleeding time, absent platelet aggregation with ristocetin but normal aggregation with ADP. The defective molecule is:

• A Glycoprotein IIb/IIIa
• B Glycoprotein Ib-IX-V complex ✓
• C von Willebrand factor itself
• D Platelet cyclooxygenase-1

Explanation

This is Bernard-Soulier syndrome, an inherited defect of the GPIb-IX-V complex, the receptor that binds von Willebrand factor and mediates platelet adhesion at injured endothelium. Loss of this receptor abolishes ristocetin-induced agglutination, produces thrombocytopenia with characteristic giant platelets, and prolongs bleeding time. Glanzmann thrombasthenia (option A) shows normal ristocetin response but absent aggregation with ADP, collagen, and epinephrine, because GPIIb/IIIa mediates fibrinogen bridging during aggregation, not adhesion.

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.