A nurse is caring for a patient who has been receiving haloperidol for schizophrenia. Which assessment finding would be the most critical indicator of neuroleptic malignant syndrome (NMS)?

When a patient on a first-generation antipsychotic like haloperidol develops agitation and fever, immediately suspect Neuroleptic Malignant Syndrome (NMS). The most critical assessment finding is severe hyperthermia, typically a core body temperature exceeding 104°F (40°C), often with diaphoresis.

This hyperthermia is not a simple fever but a sign of a hypermetabolic crisis from central dopamine blockade. It directly leads to life-threatening complications like rhabdomyolysis, metabolic acidosis, and acute kidney injury. While autonomic instability (labile blood pressure) and muscle rigidity are present, the extreme temperature elevation is the defining and most urgent indicator.