<prompt> 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 isGrams stain of ulcer dischargeDarkfield microscopy of ulcer dischargeGiemsa stain of lymph node aspirateELISA for HIV infection  <response>: It is very easy to diagnose an STD by a simple stepwise approachStep -1The first step is to see whether the ulcer is painful or painless.Painless ulcer LGV, donovanosis, chancre.Painful ulcers HSV, chancroid.Step - 2For painless ulcer, see whether lymphadenopathy is present or not:-Lymphadenopathy nt LGV, chanceNo lymphadenopathy DonovanosisIf lymphadenopathy is nt, see whether it is painful or not:-Painful lymphadenopathy LGVPainless lymphadenopathy ChancreNow see the questionStep -1An ulcer is a painless Chancre, LGV or Donovanosis.Step - 2Inguinal lymphadenopathy nt Chancre or LGV Donovanosis is ruled out.Lymphadenopathy is non-tender Diagnosis is chancre LGV is ruled out.As the clinical diagnosis is chancre Primary syphilis, Lab. diagnosis can be made by darkfield microscopy of ulcer discharge 