What event initiates the formation of a clot after an injury?

The formation of a clot after an injury is initiated by the release of thromboplastin from an injured cell. When tissue damage occurs, especially in vascular structures, cells in the affected area, such as platelets and endothelial cells, release thromboplastin (also known as tissue factor). This substance plays a crucial role in the coagulation cascade, leading to the activation of prothrombin and ultimately the conversion of fibrinogen to fibrin. This process is essential for forming a stable blood clot to prevent excessive bleeding and initiate healing.

While the constriction of blood vessels at the injury site is part of the hemostatic response that reduces blood loss, it does not directly initiate the clotting process. Other options, like the release of white blood cells or activation of plasma proteins by bacteria, are involved in inflammatory and immune responses but are not the primary triggers for clot formation. The primary event in hemostasis after injury is indeed the release of thromboplastin, making it the correct choice.