A previously healthy 8-year-old boy presents to the dermatology clinic with a progressively worsening elbow rash over the course of the last week. The rash does not itch. He spent the previous weekend sailing on the Chesapeake Bay. His pediatrician prescribed a course of cephalexin as well as a trial of topical antiviral ointment, neither of which improved the rash. The patient denies any other new exposures.