<prompt> A 26 year old male with recent history of unprotected sexual intercourse comes to opd with complaints of painless, indurated ulcer over the glans which exuded clear serum. Bilateral painless inguinal lymphadenopathy is also present. The most appropriate diagnostic test is:Gram stain of ulcer dischargeELISA for HIV infectionDark ground microscopy of ulcer dischargeGiemsa stain of lymph node aspirate  <response>: History of sexual intercourse, painless and hard ulcer chancre and painless inguinal lymphadenopathy is suggestive of primary syphilis.Dark ground examination is done to demonstrate spirochetes. 