[Syphilitic chancre in the mouth: an unusual location. Case report].

Background: Syphilis is an infectious disease caused by the spirochete Treponema pallidum, transmitted mainly by direct contact with the lesion. Primary syphilis usually presents with a chancre at the site of infection, which is highly contagious and resolves without treatment. The aim of this article is to illustrate an unusual location of a syphilitic chancre, in order to consider this diagnosis within the approach to patients with oral ulcers. Clinical case: a 30-year-old man who presented a dermatosis located in the left labial commissure, characterized by a painless ulcer of 1 cm in diameter of 20 days of evolution. The patient has a history of HIV/AIDS. A punch biopsy of the dermatosis was performed, with a histopathological report compatible with syphilitic chancre and a negative VDRL result. He was treated with penicillin G benzathine showing improvement. Conclusions: Primary syphilis is characterized by the development of the syphilitic chancre, which is the first manifestation of syphilis in up to 60% of cases. Extragenital presentation is rare, with only 12-14% of all cases, and of these between 40-70% occur in the mouth, being the lips the most frequent location. Oral manifestations can represent a diagnostic challenge due to its wide spectrum of clinical presentations.