A 26-year-old woman is seen in clinic to start HIV preexposure prophylaxis (PrEP). Baseline laboratory testing shows a negative HIV-1/2 antigen-antibody test, but a positive syphilis antibody and a rapid plasma reagin (RPR) titer of 1:32. She is asymptomatic. She has never been tested for syphilis and is thus diagnosed with latent syphilis of unknown duration. A plan is made to treat her for late latent syphilis, but she has a history of anaphylaxis after taking oral penicillin when she was 16 years of age. A urine pregnancy test is negative.