This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteriology, 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: Aztreonam is effective for Gram-negative anaerobes like Bacteroides, Porphyromonas, Prevotella. Reference: Textbook of Microbiology; Anathanarayan and paniker&;s; 9th edition; Page no: 269', 'role': 'assistant'}]