A 28-year-old male with schizophrenia was started on haloperidol 10 mg daily five days ago during a psychiatric hospitalization. The nursing staff reports he has become increasingly confused and diaphoretic over the past 24 hours. On assessment, his temperature is 104.2 degrees Fahrenheit, heart rate is 128 bpm, blood pressure is 168/102 mmHg, and respiratory rate is 24. He demonstrates severe generalized muscle rigidity described as lead-pipe in quality. Laboratory results reveal a creatine kinase of 12,400 U/L, a white blood cell count of 14,800/uL, and serum sodium of 148 mEq/L. Which assessment finding most specifically differentiates this presentation from serotonin syndrome?