A 3-week-old male neonate born to non-consanguineous parents presents with poor feeding, irritability, and a musty or 'mousy' odour on his nappies. Routine newborn screening was not performed. On examination, he is hypotonic with exaggerated deep tendon reflexes. Serum phenylalanine is 1240 µmol/L (normal <120 µmol/L), and urine phenylketones are positive. What is the most likely diagnosis?