A 32-year-old male patient is scheduled for emergency laparotomy for acute appendicitis. During rapid sequence intubation, succinylcholine 1.5 mg/kg IV is administered. Within 30 seconds of administration, the patient develops severe muscle rigidity, hyperthermia (core temperature 38.5°C), tachycardia (HR 120/min), and dark-colored urine. Serum potassium is 6.8 mEq/L (normal 3.5–5.0). What is the most likely diagnosis?