23 years 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 -

History is indicating towards the diagnosis of syphilis. T. pallidum cannot be stained by Gram stain. 1. T. pallidum can be seen from ulcer discharge by using Dark field microscopy. Dark-field microscopy:- helpful for diagnosis early in the disease before the appearance of serum antibodies. T. pallidum is identified by its slender spiral structure and cork screw motility Direct fluorescent antibody staining for T. pallidum (DFA-TP-Sensitivity is 100%) Silver impregnation method: Levaditi stain (for tissue section), Fontana stain (smear)