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:
B i.e. Dark field microscopy of ulcer discharge
Gram's stain of ulcer discharge
Darkfield microscopy of ulcer discharge
Giemsa stain of lymph node aspirate
ELISA for HIV infection
Darkfield microscopy of ulcer discharge