A 3-day-old neonate born to non-consanguineous parents presents with poor feeding, lethargy, and a characteristic musty or 'mousy' odour on the breath. Plasma phenylalanine is 1200 µmol/L (normal <120 µmol/L) and tyrosine is low. Urine shows phenylpyruvate and phenyllactate. What is the most appropriate immediate next step in management?