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
[{'content': 'Which of the following anticoagulants prevent clotting of blood in the normal vascular system: \nSelect and explain. \nA. Antithrombin III, B. Plasminogen, C. Fibrinogen, D. Heparin', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. Antithrombin III', 'role': 'assistant'}]