What is the most appropriate initial diagnostic study in a patient with a macular-papular rash and buccal ulcers indicative of secondary syphilis?

In the case of a patient presenting with a macular-papular rash and buccal ulcers suggestive of secondary syphilis, the most appropriate initial diagnostic study is the serum FTA-Absorption test. This test is a specific treponemal test that identifies antibodies to Treponema pallidum, the bacterium responsible for syphilis. The FTA-Absorption test is particularly useful after a reactive non-treponemal test, such as the RPR (Rapid Plasma Reagin) or VDRL (Venereal Disease Research Laboratory), is performed. If secondary syphilis is suspected, identifying these specific antibodies allows for a definitive diagnosis.

Other tests mentioned, such as a Tzanck smear, are used for conditions like herpes simplex virus infections and do not provide information relevant to syphilis. A culture for Streptococcus pyogenes would be appropriate for conditions related to streptococcal infections, such as pharyngitis or skin infections, but not for syphilis. An Epstein-Barr IgM titer is related to infections caused by the Epstein-Barr virus, such as infectious mononucleosis, which does not correlate with the symptoms presented in secondary syphilis