A 24-year-old male presents to a STD clinic with a single painless ulcer on external genitalia. The choice of laboratory test to look for the etiological agent would be –

Clues are:- Painless ulcer Single lesion It may be either LGV or chance:- Chancre  → Darkfield microscopy or serological tests (VDRL, FTA-ABS). IGV → Demonstration of inclusion bodies, fries test, CFT. Amongst all these tests only darkfield microscopy has been given in the options. So, I am assuming that it is a case of primary syphilis (chancre), for which dark field microscopy can be used for diagnosis.