A 42-year-old woman with treatment-resistant schizophrenia on clozapine 400 mg/day for 8 months presents with fever (38.5°C), severe muscle rigidity, altered mental status, and a serum creatinine of 2.1 mg/dL (baseline 0.9). Her CPK is 2800 U/L. Which investigation is most appropriate to confirm the suspected diagnosis of neuroleptic malignant syndrome (NMS)?