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