A 5-year-old boy with a history of recurrent ear infections is treated with a cephalosporin. Ten days later, he develops a fever of 38.5°C (101.3°F), arthralgias, and a widespread rash consisting of annular, urticarial plaques with a dusky, lavender-colored center. Laboratory evaluation is notable for the absence of circulating immune complexes and renal involvement. What is the most likely diagnosis?