A 7-year-old child develops malaise, cough, coryza, and conjunctivitis with a high fever. Prior to this illness he was completely healthy. Further questioning reveals that he has not received any childhood vaccinations because of his parents' religious beliefs. Examination of his mouth reveals blue white spots on a red base beside his second molars. The next day he develops an erythematous, nonpruritic, maculopapular rash at his hairline and behind his ears, which spreads over his body. Which of the following is the most likely diagnosis?