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': 'CMV retinitis in HIV occurs when the CD4 counts falls below - \nSelect and explain. \nA. 50, B. 100, C. 200, D. 150', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]