A 6-month-old female infant presents with poor feeding, vomiting, and lethargy that worsens after milk feeds. On examination, she has hepatomegaly and a musty odor to her urine. Newborn screening was not performed. Serum amino acid analysis shows elevated phenylalanine (1200 µmol/L; normal <120 µmol/L) and elevated tyrosine. Urine organic acids show phenylpyruvate. Which of the following is the most appropriate immediate management?