A 28-year-old woman with a known history of partial ornithine transcarbamylase (OTC) deficiency presents to the emergency department with acute encephalopathy, confusion, and a plasma ammonia level of 220 µmol/L. Her neuroimaging is normal. To differentiate acute hyperammonemia from other causes of encephalopathy and confirm the urea cycle disorder as the culprit, which investigation is most specific?