A 24-year-old woman is seen for an initial pregnancy visit. She is estimated to be at 15 weeks of gestation. Her physical examination is normal, and she has no neurologic, ocular, or otic signs or symptoms. Routine laboratory screening tests are performed, including a serologic test for syphilis. The positive treponemal enzyme immunoassay (EIA) is reactive, and the rapid plasma reagin (RPR) assay is positive with a titer of 1:16. The HIV-1/2 antigen-antibody test is negative. She had testing for sexually transmitted infections 9 months ago, including serologic testing for syphilis; all tests were negative. She returns to the clinic for treatment of early latent syphilis, but she tells you she developed hives when she received oral penicillin about 3 years ago.