Clinicopathological, dermoscopic, and ultrastructural observation of annular secondary syphilis on the penis.

Annular syphilis may range from mildly raised lesions with scaly borders to verrucous plaques. Localized annular syphilis on the genitalia has been rarely reported in HIV-negative cases. This paper reports a case of annular secondary syphilis on the penis. Dermoscopy showed peripheral dotted and short linear vessels and white scaling with a relatively clear central area in an erythematous annular plaque. Histopathology revealed mild hyperkeratosis, parakeratosis, psoriasiform acanthosis, and focal basal vacuolar degeneration with lichenoid, perivascular, and periadnexal infiltrate of lymphohistiocytes and plasma cells in the superficial dermis. Silver stain showed several spirochetes in the lower epidermis and superficial dermis. Electron microscopy revealed a few intercellular and intracytoplasmic spirochetes in the basal epidermis and free spirochetes in the papillary dermis. Rapid plasma reagin and Treponema pallidum particle agglutination assays were positive. The lesions disappeared after intramuscular benzathine penicillin, with no relapse at six-month follow-up.