A 4-yeаr-оld child is brоught tо the clinic with а 4-dаy history of fever, cough, runny nose, and red watery eyes. Physical examination reveals small white lesions on the buccal mucosa. Twenty-four hours later, the child develops a red maculopapular rash that begins along the hairline and behind the ears before spreading downward to the trunk and extremities. Which of the following is the most likely diagnosis?