A 27-year-old man develops a painless 1-cm sore on his penis. It appears ulcerated with a raised margin and minimal serous exudates. (See Figure below) Which of the following is the most appropriate next step in the diagnosis?

This is likely a case of primary syphilis. Serology is the most appropriate test; it can remain negative for a period up to 1 month after the infection is contracted. The serologic test for syphilis usually is positive within 1 week after the chancre appears. With therapy, the chancre heals rapidly, but will heal in 4-6 weeks even without treatment. Genital chancres are usually painless, unless super-infected, but extragenital chancres (eg, fingers) can be quite painful. Biopsy is usually not necessary for diagnosis, and the spirochetes are seen with dark-field examination, not with Gram stain.