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: Dental
[{'content': "A 23-year-old male had unprotected sexual intercourse with a commercial sex worker. Two weeks later, he developed a painless, indurated ulcer on the glans which 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. Darkfield microscopy of ulcer discharge, C. Giemsa stain of lymph node aspirate, D. ELISA for HIV infection", 'role': 'user'}, {'content': 'Answer: B \nExplanation: It is very easy to diagnose an STD by a simple stepwise approach\n\n\u200bStep -1\n\nThe first step is to see whether the ulcer is painful or painless.\nPainless ulcer\xa0\xa0\xa0\xa0\xa0→\xa0\xa0 LGV, donovanosis, chancre.\nPainful ulcers\xa0\xa0\xa0\xa0→ HSV, chancroid.\n\nStep - 2\n\nFor painless ulcer, see whether lymphadenopathy is present or not:-\nLymphadenopathy (+)nt\xa0→ LGV, chance\nNo lymphadenopathy\xa0\xa0\xa0→\xa0 Donovanosis\nIf lymphadenopathy is (+)nt, see whether it is painful or not:-\nPainful lymphadenopathy → LGV\nPainless lymphadenopathy → Chancre\n\t\u200b\n\nNow see the question\nStep -1\nAn ulcer is a painless → Chancre, LGV or Donovanosis.\n\nStep - 2\nInguinal lymphadenopathy (+)nt →\xa0Chancre or LGV (Donovanosis is ruled out).\nLymphadenopathy is non-tender → Diagnosis is chancre (LGV is ruled out).\n\nAs the clinical diagnosis is chancre (Primary syphilis), Lab. diagnosis can be made by darkfield microscopy of ulcer discharge', 'role': 'assistant'}]