Ans. c. von-Willebrand factor (Ref: Robbins 9/e p117,118. 8/e p116-118)Platelet adhesion to collagen occurs via von- Willebrand factor)'Endothelial injury allows platelets to contact the underlying extracellular matrix; subsequent adhesion occurs through interactions with von Willebrand factor (vWF), which is a product of normal endothelial cells and an essential cofactor for platelet binding to matrix elements.' -Robbin 8/e p116Platelet AdhesionCertain proteins are expressed on the platelet surface that subsequently regulate collagen-induced platelet adhesion particularly under flow conditions, and include glycoprotein (GP)-IV, GP-V1. and the integrin-alfa2, and beta1.The GPIb-IX-V complex binds to the exposed von-Willebrand factor, causing platelets to adhere.Von-Willebrand factor--bound GPlb-IX-V promotes a calcium-dependent conformational change in the GPlIb/IIIa receptor, transforming it from an inactive low-affinity state to an active high-affinity receptor for fibrinogen.Factors Promoting Platelet Aggregation* ADPQ* EpinephrineQ* Thromboxane-A2Q* Serotonin* von-Willebrand factorQ* FibrinogenQ* ThrombospondinQ* Collagen* Immune complex* ThrombinQ