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': 'A darkfield microscope would be useful in examining blood for: \nSelect and explain. \nA. Treponema, B. Actinomyces, C. Streptococcus, D. Mycobacterium', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]