A 32-year-old male undergoes elective orthopedic surgery under general anesthesia with propofol and rocuronium. Within 20 minutes of induction, the anesthesiologist notes masseter muscle rigidity, a sharp rise in end-tidal CO₂ (ETCO₂) from 35 to 55 mmHg, and a core temperature of 38.5°C. Which finding best distinguishes this presentation as malignant hyperthermia rather than sepsis or thyroid storm?