This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
[{'content': 'The blood within the vessels does not clot normally because \nSelect and explain. \nA. Vitamin K antagonists are present in plasma, B. Thrombin has a positive feedback on plasminogen, C. Sodium citrate in plasma chelates calcium ions, D. Vascular endothelium is smooth and coated with glycocalyx', 'role': 'user'}, {'content': "Answer: D \nExplanation: The endothelium of the blood vessels also plays an active role in preventing the extension of clots. All endothelial cells except those in the cerebral microcirculation produce thrombomodulin, a thrombin-binding protein, on their surfaces. InCirculating blood, thrombin is a procoagulant that activates factors V and VIII, but when it binds to thrombomodulin, it becomes an anticoagulant in that the thrombomodulin-thrombin complex activates protein C. Activated protein C (APC), along with its cofactor protein S, inactivates factors V and VIII and inactivates an inhibitor of tissue plasminogen activator, increasing the formation of plasmin. Ref: Ganong's Review of Medical Physiology Twenty-Third Edition Page No: 533", 'role': 'assistant'}]