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 agents in not likely to be found in plasma? \nSelect and explain. \nA. Thrombin, B. Fibrinogen, C. Prothrombin, D. Calcium ion', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]