Advanced Dental Admission Test (ADAT)

What initiates the coagulation cascade?

1. The release of calcium ions

2. A tissue factor binding to factor VIIa

3. The activation of prothrombin

4. The exposure of collagen fibers

The correct answer is: A tissue factor binding to factor VIIa

The coagulation cascade is initiated by the binding of tissue factor (TF) to factor VIIa. Tissue factor is a glycoprotein present on the surface of subendothelial cells and is released upon vascular injury. When the vascular endothelium is disrupted, tissue factor comes into contact with circulating factor VII, converting it to its active form, factor VIIa. The tissue factor-factor VIIa complex then activates factor X, which is a crucial step in the common pathway of coagulation leading to the formation of thrombin and subsequently fibrin. This process is essential for hemostasis, as it leads to the formation of a stable blood clot to prevent excessive bleeding. The initiation of the cascade through the tissue factor-factor VIIa interaction underscores the significance of vascular injury in the coagulation process, making this option the most accurate in reflecting the initial step of coagulation. While factors such as calcium ions, collagen exposure, and prothrombin activation are all important in subsequent steps of coagulation, they do not serve as the primary initiators.