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: Microbiology
[{'content': 'Effective for common gram-negative anaerobes ? \nSelect and explain. \nA. Aztreonam, B. Doxycycline, C. Vancomycin, D. Tobramycin', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Aztreonam", 'role': 'assistant'}]