A 32-year-old male patient is scheduled for emergency intubation in the ICU. He receives succinylcholine 1.5 mg/kg IV for rapid sequence induction. Within 2 minutes of administration, he develops severe hyperkalemia (K⁺ 7.2 mEq/L), followed by cardiac arrhythmias and cardiac arrest. The anesthesiologist suspects a genetic predisposition to abnormal succinylcholine metabolism. Which investigation is most appropriate to confirm the suspected diagnosis?