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: Pathology
[{'content': 'Which is must for prothrombin time? \nSelect and explain. \nA. Thromboplastin, B. Prothrombin, C. Fibrinogen, D. Fibrin', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Prothrombin time test - Time needed for plasma to clot after addition of tissue thromboplastin and Ca2+\xa0ions.', 'role': 'assistant'}]