This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteria, Subject name: Microbiology
[{'content': "23 years 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: Ans. is 'b' i.e., Dark field microscopy of ulcer discharge (Ref: Harrison, 17th/e, p. 1384 ,18th/e, p. 1043 and19th/e, p. 1132; Ananthanarayan, 9th/e, p. 370)History is indicating towards syphilis and T. pallidum can be seen from ulcer discharge by using Dark field microscopy.", 'role': 'assistant'}]