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 –

It is very easy to diagnose an STD by a simple stepwise approach ​Step -1 The first step is to see whether the ulcer is painful or painless. Painless ulcer     →   LGV, donovanosis, chancre. Painful ulcers    → HSV, chancroid. Step - 2 For painless ulcer, see whether lymphadenopathy is present or not:- Lymphadenopathy (+)nt → LGV, chance No lymphadenopathy   →  Donovanosis If lymphadenopathy is (+)nt, see whether it is painful or not:- Painful lymphadenopathy → LGV Painless lymphadenopathy → Chancre ​ Now see the question Step -1 An ulcer is a painless → Chancre, LGV or Donovanosis. Step - 2 Inguinal 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