A 2-year-old girl from Mumbai with a history of failure to thrive and developmental delay presents with acute onset of irritability, poor feeding, and lethargy. Serum ammonia is 240 µmol/L. Plasma amino acid profile shows markedly elevated lysine, arginine, and ornithine with low citrulline and argininosuccinate. Urine shows elevated lysine, ornithine, and cystine. What is the most appropriate immediate next step?