Von Willebrand factor is best described as which of the following?

• A. Phospholipid required for multiple reactions in the coagulation sequence
• B. Plasma protein that binds platelets to exposed subendothelial collagen
• C. Plasma protein with procoagulant activity in the intrinsic coagulation system
• D. Platelet membrane glycoprotein that attaches the platelet to the injured vessel wall

Answer: (B)

Von Willebrand factor mediates primary hemostasis by anchoring platelets to damaged vessel wall. It is a plasma glycoprotein that binds exposed subendothelial collagen and simultaneously binds the platelet receptor GPIb-IX-V, forming a bridge that tethers platelets to the site of injury and promotes the formation of the platelet plug. It also stabilizes circulating factor VIII, protecting it from degradation and delivering it to the injury site. It is not a phospholipid (that would be a lipid molecule), not a procoagulant enzyme in the intrinsic pathway, and not a platelet membrane glycoprotein itself—though it interacts with platelet receptors. This makes the description of a plasma protein that binds platelets to exposed subendothelial collagen the best fit.