The diagnosis of syphilis cannot be made simply by virtue of the erythema on the inner thigh.
Whether there is a history of unclean sexual contact.
Whether there are serological manifestations of syphilis (TPPA; PRP, TRUST, etc.).
Whether there are characteristic manifestations of syphilis such as hard chancre (stage I), sclerosing lymphadenitis (stage I), flat warts (stage II), palmoplantar syphilis rash (stage II), syphilitic alopecia (stage II), and nodular syphilis rash (stage III).