Secondary syphilis as a single annular plaque on the penis mimicking granuloma annulare
A 44-year-old homosexual man currently on antiretroviral therapy for HIV infection presented with a three month history of a single annular red plaque of two cm diameter with central clearing and raised borders on the penis, associated with mild pruritus [Figure 1]. There were no lesions in the oral cavity, palms, or soles. Lymphadenopathy and systemic symptoms were absent.
Histopathological examination revealed a dense dermal infiltrate composed of lymphocytes and plasma cells in a lichenoid pattern with epitheliotropism. Immunohistochemistry showed the presence of Treponema pallidum. Rapid plasma reagin (RPR) test was reactive in 1:32 dilution and fluorescent treponemal antibody absorption test was positive. The CD4 lymphocyte count was within normal limits. A diagnosis of secondary syphilis was made and the patient received 2.4 million units of penicillin G benzathine weekly for 3 weeks, despite the existing guidelines preferring a single-dose therapy, leading to a complete healing of the lesion. Serology performed 3 months after treatment showed an improved RPR of 1:4 dilution.