A 22-year-old man is diagnosed with secondary syphilis based on a diffuse maculopapular rash and a Rapid Plasma Reagin (RPR) titer of 1:256. In the clinic, he is given an intramuscular dose of 2.4 million units of benzathine penicillin G. Approximately 4 hours after receiving the dose of penicillin, he returns with fever, headache, myalgia, dizziness, and intensification of his rash. Physical examination shows a temperature of 38.5°C, blood pressure of 96/74 mmHg, accentuation of the rash, but no evidence of hives, oropharyngeal swelling, or wheezing.