Final answer:
The extrinsic pathway, triggered by trauma, is the pathway leading to fibrin formation that is initiated by a substance not normally found in the blood.
Step-by-step explanation:
The extrinsic pathway is the pathway leading to fibrin formation that is initiated by a substance that is not a normal component of the blood. This pathway is triggered by trauma, such as damage to the surrounding tissues in a traumatic injury. When blood plasma comes into contact with damaged extravascular cells, factor III (thromboplastin) is released, which activates the extrinsic pathway. This pathway then activates factor X, leading to the common pathway and the formation of a blood clot.