A 22-year-old woman is brought from a music festival by paramedics. She is agitated and confused, her temperature is 39.8 °C, pulse 132/min, blood pressure 158/94 mmHg and her skin is flushed and dripping with sweat. Her pupils are 7 mm and reactive. Her limbs are tremulous, the knee and ankle reflexes are very brisk, and tapping the sole produces a rhythmic beating of the ankle that continues for more than 10 beats. Friends say she "took a pill" two hours ago. Her regular medication is sertraline 100 mg daily.
Which one of the following best explains her presentation?