A 28-year-old female taking fluoxetine 40 mg daily presents to the emergency department after her primary care provider prescribed sumatriptan for a severe migraine earlier today. She now presents with agitation, diaphoresis, and tremor. On examination, vital signs reveal temperature 38.9°C, heart rate 112 bpm, and blood pressure 162/98 mmHg. Neurological examination reveals bilateral lower extremity clonus greater in the legs than arms, hyperreflexia, and mydriasis. She is oriented but restless with intermittent myoclonic jerks. Which assessment finding most reliably distinguishes serotonin syndrome from neuroleptic malignant syndrome in this clinical presentation?