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: Surgery
[{'content': 'Hyperacute graft rejection is caused by ? \nSelect and explain. \nA. Preformed antibodies, B. T- lymphocytes, C. Macrophages, D. B - lymphocytes', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Preformed antibodies", 'role': 'assistant'}]