A 6-year-old boy from rural Maharashtra presents with a 3-day history of high fever (39.5°C), cough, coryza, and conjunctivitis. On examination, he has a blanching maculopapular rash that started on the forehead and hairline and has now spread to the trunk and limbs. The rash spares the palms and soles. On intraoral examination, you note small white spots with red halos on the buccal mucosa opposite the molars. His mother reports he is unvaccinated. What is the most likely diagnosis?