The coagulation cascade is initiated by which of the following?

The coagulation cascade is initiated primarily by the exposure of collagen and tissue factor. When blood vessels are injured, the underlying collagen in the vascular wall becomes exposed to the bloodstream. This exposure triggers the initial steps of the coagulation process. Tissue factor (also known as thromboplastin), which is present in tissues and is released by damaged cells, binds with Factor VII, leading to the activation of the extrinsic pathway of coagulation. This activation is essential for the subsequent series of reactions that ultimately lead to thrombin generation and fibrin formation, essential components of blood clotting.

While calcium is important in the coagulation process, specifically as a cofactor for several coagulation factors, it is not the initial trigger that starts the cascade. The aggregation of platelets does occur later in the process as they respond to the activation signals, and the formation of the fibrin mesh occurs at a later stage after the initial cascade has been initiated and amplified. Thus, the correct focus on the initiation of the coagulation cascade revolves around the exposure of collagen and tissue factor.