Clinical Challenge: Why Would This Word Appear on This Man's Back?

A 20-year-old college student presents with intermittent itching of approximately 4 weeks duration. He is in good health and currently takes no oral medications. He relates treatment of a sinus infection with amoxicillin several days prior to the onset of his pruritus. Full-body examination is unremarkable and no hives are apparent. Stroking of the skin of his back elicits raised ...