A 23year 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 -
Gram's stain of ulcer discharge Dark field microscopy of ulcer discharge Giemsa stain of lymph node aspirate ELISA for HIV infection