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 23year old male had unprotected sexual intercourse with a commercial sex worker. Two weeks later, he developed a painless, indurated ulcer on the glans that exuded clear serum on pressure. Inguinal lymph nodes in both groins were enlarged and not tender. The most appropriate diagnostic test is - \nSelect and explain. \nA. Gram's stain of ulcer discharge, B. Dark field microscopy of ulcer discharge, C. Giemsa stain of lymph node aspirate, D. ELISA for HIV infection", 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]