A two-year-old female has had a fever for one week. The patient has been lethargic and has had a poor appetite. On examination, the child appears ill with a temperature of 38.7°C. The patient has a diffuse, macular, erythematous rash and her fingertips are peeling. Her tongue is bright red with white papillae, her oropharynx is injected, and there is cervical lymphadenopathy. Which of the following is the most likely diagnosis?