Q. A 30-year-old man presents to your clinic with complaints of fever, chills, headache, and myalgias. He reports that these symptoms started within a few hours after receiving treatment for a sexually transmitted infection (STI). On further questioning, he mentions that he was diagnosed with syphilis and received a single dose of penicillin at another clinic. His past medical history is unremarkable, and he denies any known drug allergies. His temperature is 101.3°F (38.5°C), pulse is 92 beats per minute, blood pressure is 130/80 mmHg, and respiratory rate is 18 per minute. Physical examination shows a penile chancre. What is the most likely explanation for the patient's current symptoms?