Which step in hemostasis forms a temporary seal?

Platelet plug formation is the step in hemostasis that forms a temporary seal at the site of vascular injury. This occurs as part of primary hemostasis: when a vessel is damaged, exposed collagen and von Willebrand factor attract platelets, which adhere to the injury site. The platelets then become activated, releasing chemicals such as ADP and thromboxane A2 that recruit additional platelets. These aggregating platelets form a loose, temporary platelet plug that quickly seals the break in the vessel wall. This plug is temporary and must be reinforced by coagulation. Coagulation, also known as secondary hemostasis, involves the clotting cascade that converts fibrinogen to fibrin, which strengthens and stabilizes the platelet plug into a more permanent clot. Vascular spasm is actually the first step in hemostasis — it involves immediate vasoconstriction to reduce blood flow to the injured area, but it does not form a physical seal. Clot retraction occurs later, when platelet contractile proteins compress the clot to pull wound edges together and squeeze out serum. Only platelet plug formation directly creates the physical temporary barrier that stops initial bleeding, making it the correct answer.