This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Extrinsic system (blood coagulation) is triggered by the release of: September 2008 
Select and explain. 
A. Prothrombin, B. Thromboplastin, C. Fibrinogen, D. Thrombin
B: Thromboplastin
Answer: B: Thromboplastin 
Explanation: Ans. B: Thromboplastin Factor X can be activated by reactions in either of two systems, an intrinsic and an extrinsic system. The extrinsic system is triggered by the release of tissue thromboplastin, a protein-phospholipid mixture that activates factor VII. The tissue thromboplastin and factor VII activate factors IX and X. In the presence of PL, Ca2+, and factor V, activated factor X catalyzes the conversion of prothrombin to thrombin. The extrinsic pathway is inhibited by a tissue factor pathway inhibitor that forms a quaternary structure with TPL, factor VIIa, and factor Xa.