1. A 7-year-old boy is brought to a physician because of a nearly confluent, fine, erythematous, macular rash that is most pronounced on his trunk. He has had a mild fever for 36 hours, but does not appear very sick, and is happily playing when the pediatrician enters the examining room. Physical examination demonstrates a reddened throat with tonsillar exudates, enlarged cervical nodes including the occipital node, and questionable splenomegaly. The mother says that the boy has not been coughing, and no Koplik spots are noted. Which of the following is the most likely diagnosis?