A 23-year-old man who is receiving tenofovir DF-emtricitabine for HIV preexposure prophylaxis (PrEP) is seen for a routine follow-up visit. Screening laboratory tests show a positive treponemal enzyme immunoassay (EIA) and a positive Rapid Plasma Reagin (RPR), with a titer of 1:32. Tests for HIV and other sexually transmitted infections (STIs) are negative. He does not have any symptoms and does not recall any symptoms during the past year that were consistent with syphilis. He states he has never been diagnosed with syphilis, nor has he received treatment for syphilis in the past. He was screened with a treponemal EIA test 4 months ago, which was negative. A physical examination does not show any abnormalities. He does not have any antibiotic allergies.