In a platelet aggregation study, the absence of aggregation in response to ristocetin indicates which condition?

In a platelet aggregation study, the absence of aggregation in response to ristocetin indicates Bernard-Soulier syndrome. This condition is characterized by a defect in the platelet glycoprotein (primarily glycoprotein Ib-IX-V), which is essential for the binding of platelets to von Willebrand factor. Ristocetin acts to facilitate this interaction, so when the glycoprotein is dysfunctional, the expected aggregation response does not occur.

The other options relate to different aspects of platelet function or aggregation. Von Willebrand disease typically shows reduced aggregation with ristocetin but may still display some level of aggregation depending on the severity. Thrombasthenia, which involves a defect in glycoprotein IIb/IIIa, usually results in a lack of aggregation with other aggregating agents but may respond to ristocetin initially. Aspirin-induced platelet dysfunction affects platelet thromboxane A2 production, leading to impaired aggregation, but does not specifically cause a lack of response to ristocetin. Thus, the specific lack of aggregation in response to ristocetin is most indicative of Bernard-Soulier syndrome.