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: ENT
[{'content': 'Commonest cause of brain abscess - \nSelect and explain. \nA. CSOM, B. Pyogenic meningitis, C. Trauma, D. Chr. sinusitis', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]