A 12-year-old boy with chronic, unexplained hepatitis is being evaluated for Wilson disease. His serum ceruloplasmin level is 15 mg/dL (normal > 20 mg/dL), and a 24-hour urine collection shows copper excretion of 180 µg/day (normal < 40 µg/day). A slit-lamp examination by an ophthalmologist is negative for Kayser-Fleischer rings. What is the most definitive test to confirm the diagnosis?