UCP2.04 Bad Blood Practice Test 2026 – Complete Exam Prep

Factor III is tissue factor, also known as tissue thromboplastin. This substance is released by damaged tissues and acts as the trigger for the extrinsic pathway of coagulation. When tissue factor encounters circulating Factor VIIa in the presence of calcium, it forms a complex that activates Factor X (and IX to some extent), jumping the cascade toward thrombin generation and fibrin clot formation.

Calcium is Factor IV and serves as a necessary cofactor in several steps, but it is not the initiating substance. Proaccelerin, also called labile factor, is Factor V, a cofactor in the common pathway, not the initiator. Fibrinogen is Factor I, the soluble precursor that becomes fibrin, not the initiating trigger. Hence tissue thromboplastin best matches Factor III.