A 6-month-old girl, born to consanguineous parents in Delhi, presents with poor feeding, developmental delay, and recurrent vomiting after milk feeds. She has mild intellectual disability and a musty odour to her urine. Serum ammonia is 210 μmol/L (normal <50). Plasma amino acid profile shows markedly elevated arginine (450 μmol/L; normal 40–130) and elevated lysine. Urine shows elevated arginine and lysine. Liver function tests are normal. Genetic testing confirms a homozygous mutation in the AGXT2 gene. Which urea cycle intermediate is most likely elevated in this patient's plasma?