18. Prothrombin is converted to thrombin during the __ 1. intrinsic pathway 2. extrinsic pathway 3. common pathway 4. formation of the platelet plug

## Step 1<br />Prothrombin, also known as factor II, is a protein that plays a crucial role in blood clotting. It is converted into thrombin, which is a key enzyme in the clotting process.<br /><br />## Step 2<br />The conversion of prothrombin to thrombin occurs in the common pathway of the coagulation cascade. The coagulation cascade is a series of events that lead to the formation of a blood clot. It consists of three main pathways: the intrinsic pathway, the extrinsic pathway, and the common pathway.<br /><br />## Step 3<br />The intrinsic pathway is activated by damage to the blood vessel's inner lining. The extrinsic pathway is activated by external trauma that causes blood to escape from the vessel. The common pathway is the final stage of the coagulation cascade, where the intrinsic and extrinsic pathways converge.<br /><br />## Step 4<br />The formation of the platelet plug is a separate process that occurs before the coagulation cascade. It involves the activation of platelets, which adhere to the site of injury and form a plug to prevent further bleeding.