A nurse is assessing a client who takes citalopram to treat depression. Which of the following findings should the nurse identify as an indication of serotonin syndrome?

Choice A reason: Muscle rigidity is a hallmark symptom of serotonin syndrome, often accompanied by hyperreflexia, tremors, and autonomic instability. It requires immediate intervention. Choice B reason: Constipation is not associated with serotonin syndrome. It may be a side effect of other medications but is not diagnostic. Choice C reason: Lethargy is nonspecific and may occur in many conditions. Serotonin syndrome typically presents with agitation and hyperactivity rather than sedation. Choice D reason: Bradycardia is not typical of serotonin syndrome. Tachycardia and hypertension are more common due to autonomic dysregulation.