A 17-year-old was healthy until he developed an upper respiratory tract infection. This was followed three days later with a high-grade fever, malaise, and a petechial skin rash. In the emergency department, he had a blood pressure of 70/40 mmHg, a heart rate of 120 beats per minute, and a white blood cell count of 22. Labs showed sodium 120 mEq/L, potassium 5.8 mEq/L, chloride 89 mEq/L, blood urea nitrogen 34 mg/dL, creatinine 2.0 mg/dL, and glucose 50 mg/dL. Which of the following diagnoses is most likely?