• Autosomal recessive. Incidence ≈ 1:10,000.
  • Pathophysiology
    • ↓ phenylalanine hydroxylase or
    • ↓ tetrahydrobiopterin (BH4) cofactor (malignant PKU): ↑ tryptophan & ↓ serotonin → neurologic deterioration.
    • See Catecholamine synthesis
    • ↑ phenylalanine → Phenylalanine transaminated to phenyl pyruvic acid, or decarboxylated to phenylethylamine, disrupts normal metabolism & causes brain damage; also produces excess phenyl ketones in urine.
    • Phenyl ketones—phenylacetate, phenyllactate, and phenylpyruvate.
    • PKU patients must avoid the artificial sweetener aspartame, which contains phenylalanine.
  • Findings:
    • Normal at birth, followed by gradual intellectual disability, growth retardation
    • Infant: severe vomiting, hypertonicity, hyperactive DTRs, seizures
    • Older: hyperactive with purposeless movements, rhythmic rocking & athetosis
    • Fair complexion, eczema
    • Disorder of aromatic amino acid metabolism → musty body odor.
    • Maternal PKU—lack of proper dietary therapy during pregnancy. Findings in infant: microcephaly, intellectual disability, growth retardation, congenital heart defects.
  • Treatment: ↓ phenylalanine and ↑ tyrosine (becomes essential) in diet, tetrahydrobiopterin supplementation.
  • Screening: 2–3 days after birth (normal at birth because of maternal enzyme during fetal life).

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