Glanzmann thrombasthenia is defined by deficiency of which platelet receptor?

• A. GPIIb/IIIa
• B. GPIIb-IX-V
• C. GPVI
• D. GPIa/IIa

Answer: (A)

Glanzmann thrombasthenia is a platelet aggregation defect caused by deficiency of the platelet receptor GPIIb/IIIa (integrin αIIbβ3). This receptor on activated platelets binds fibrinogen (and von Willebrand factor) to cross-link platelets and form the platelet plug. Without GPIIb/IIIa, the fibrinogen bridges needed for platelets to aggregate are missing, so aggregation in response to most activators (ADP, epinephrine, collagen, thrombin) is severely impaired. The ristocetin pathway, which relies on GPIb-VWF rather than GPIIb/IIIa, remains functional, so ristocetin-induced aggregation is preserved. This distinction helps differentiate Glanzmann from other platelet disorders, such as Bernard-Soulier, which involves GPIb-IX-V deficiency affecting adhesion rather than aggregation.