Secondary Syphilis as a Mimic of Cutaneous Small-Vessel Vasculitis: A Diagnostic Pitfall.

Secondary syphilis is renowned for its ability to mimic a wide range of dermatologic and systemic diseases, including vasculitides. Misinterpretation of its cutaneous manifestations can delay appropriate therapy and expose patients to unnecessary immunosuppression. We report the case of a young female who presented with an acute, maculopapular, and annular rash initially affecting the upper body, including the back, chest, and upper limbs, followed by involvement of the lower body. It was preceded by redness and inflammation of her eyes. Initial clinical evaluation supported a diagnosis of cutaneous small-vessel vasculitis, and systemic vasculitis was strongly suspected. However, an extended history revealed recent unprotected sexual contact a few weeks before the symptoms started. Serologic testing demonstrated positive non-treponemal and treponemal tests consistent with secondary syphilis. The patient was treated with intravenous ceftriaxone, resulting in rapid resolution of the rash and improvement of her symptoms, with an appropriate decline in non-treponemal titers on follow-up. This case illustrates how secondary syphilis can present with a vasculitis-appearing rash clinically and underscores the need to include syphilis in the diagnostic workup of small-vessel vasculitis. Routine sexual history-taking and early syphilis serology in adults with an unexplained vasculitis rash may prevent diagnostic delay, inappropriate immunosuppression, and ongoing transmission.