Which of the following is a symptom of serotonin syndrome?

Restlessness is indeed a symptom of serotonin syndrome. This condition typically arises from an excess of serotonin in the central nervous system, often due to the use of certain medications, including antidepressants, opioids, or supplements that increase serotonin levels. A hallmark of serotonin syndrome is a heightened state of arousal, which can manifest as restlessness or agitation. Individuals may feel extremely uneasy, unable to sit still, or exhibit an increased level of nervous energy.

The other symptoms listed may not align with the typical presentations of serotonin syndrome. For instance, rapid weight gain is not a common symptom associated with this condition, nor is severe insomnia, although overactivity might cause restless or disrupted sleep. Bradycardia, which refers to a slow heart rate, is also generally not characteristic of serotonin syndrome; instead, tachycardia, or elevated heart rate, is more prevalent due to increased sympathetic activity. Thus, restlessness stands out as a defining symptom in this context.