An 18-year-old man with HIV is seen in clinic for a routine follow-up visit. He reports being sexually active with men. He presents with a diffuse maculopapular rash. He has a positive treponemal antibody test and a serum rapid plasma reagin (RPR) titer of 1:256. He is diagnosed with secondary syphilis. He reports that he had facial swelling and hives when he received amoxicillin about 4 years ago.