A 6-year-old child presents with a 2-day history of low-grade fever, malaise, and a maculopapular rash that began on the face and spread downward over 24 hours. On intraoral examination, the structure marked **A** is noted — pinpoint rose-red petechial macules on the soft palate. Tender lymphadenopathy is palpable in the posterior auricular and suboccipital regions. Which of the following is the most likely diagnosis?