A 32-year-old male underwent elective open cholecystectomy under general anesthesia with succinylcholine and isoflurane. Intraoperatively, at 45 minutes, the patient developed unexplained tachycardia (HR 140/min), muscle rigidity, and core temperature rose to 38.5°C. Capnography showed elevated end-tidal CO₂ (ETCO₂ 65 mmHg). The anesthesiologist immediately discontinued triggering agents and hyperventilated with 100% O₂. Which investigation is most appropriate to confirm the diagnosis of malignant hyperthermia at this critical moment?