This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.N.S, Subject name: Medicine
[{'content': 'Most common cause of epidural abscess \nSelect and explain. \nA. Staphylococcus, B. Streptococcus, C. Gram negative bacilli, D. Microanerophilic anaerobic streptococci', 'role': 'user'}, {'content': "Answer: A \nExplanation: Harrison's principles of internal medicine. * Staphylococci or gram negative organisms are the usual cause of an epidural abscess", 'role': 'assistant'}]