<prompt> 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:Grams stain of ulcer dischargeDarkfield microscopy of ulcer dischargeGiemsa stain of lymph node aspirateELISA for HIV infection  <response>: B i.e. Dark field microscopy of ulcer discharge 