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Key Features

  • Mental retardation, hyperactivity, seizures, light complexion, and eczema characterize untreated patients

  • Newborn screening for elevated plasma phenylalanine identifies most infants

  • Disorders of cofactor metabolism also produce elevated plasma phenylalanine level

  • Early diagnosis and treatment with phenylalanine-restricted diet prevents mental retardation

  • an autosomal recessive trait, with an incidence in whites of approximately 1:10,000 live births

Clinical Findings

  • There is little or no phenylalanine hydroxylase activity

  • Patients with untreated phenylketonuria exhibit severe mental retardation, hyperactivity, seizures, a light complexion, and eczema


  • Infants should be screened during the first few days

  • A second test is necessary when newborn screening is done before 24 hours of age, and should be completed by the third week of life

  • Plasma phenylalanine and phenylalanine/tyrosine ratio are elevated in a child on a normal diet

  • Plasma phenylalanine levels are persistently elevated above 1200 μM (20 mg/dL) on a regular diet, with normal or low plasma levels of tyrosine, and normal pterins


  • Dietary restriction of phenylalanine

  • Phenylalanine levels should be maintained at less than 360 μM (6 mg/dL)

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