What is the coagulation factor involved exclusively in the extrinsic coagulation system? SELECT ONE.

There are two coagulation pathways - extrinsic and intrinsic. In the extrinsic system, tissue thromboplastin (a lipoprotein) interacts with factor VII. The intrinsic pathway requires factors XII, XI, IX, and VIII. Factor XII is the initial step in the coagulation cascade. Factor XII, activated by contact with a nonendothelial substance, will activate factor XI (plasma thromboplastin antecedent). However, factor XI can be activated even when factor XII is deficient. Calcium is required for nearly all of the enzyme reactions in both the intrinsic and extrinsic systems. The amount of ionized calcium required for these reactions is extremely small, and clinical hypocalcemia itself is not a cause of abnormal bleeding. Fibrin split products are not part of the normal pathway in either the intrinsic or extrinsic system. The excessive breakdown of fibrinogen results in measurable amounts of the breakdown products of fibrinogen in the blood. Their presence may signal DIC if the PT and platelet count are deranged. In pure fibrinolysis, fibrinogen breakdown product levels also may be increased.