Which statement describes a hallmark of serotonin syndrome?

• A. Hyperreflexia with clonus and autonomic instability.
• B. Hyporeflexia and rigidity.
• C. Muscle atrophy with neuropathy.
• D. Weight gain with edema.

Answer: (A)

Serotonin syndrome stems from excess serotonin triggering a rapid surge of autonomic and neuromuscular activity. The defining feature is neuromuscular hyperreactivity, especially hyperreflexia with clonus, occurring together with autonomic instability such as tachycardia, hypertension, fever, and diaphoresis. This combination of exaggerated reflexes and autonomic signs is the hallmark of the toxidrome, making this statement the best description.

The other options describe patterns that don’t fit this acute serotonergic picture: hyporeflexia with rigidity is more typical of neuroleptic malignant syndrome, muscle atrophy with neuropathy points to chronic degenerative neuropathies, and weight gain with edema isn’t characteristic of the acute serotonergic syndrome.