250 pmol/L (4 mg/dL).# Treatment consists of a special diet low in phenylalanine and supplemented with tyrosine, since tyrosine becomes an essential amino acid in phenylalanine hydroxylase deficiency.# With therapy, plasma phenylalanine cone should be maintained between 120 and 360 pmol/L (2 and 6 mg/dL).# Dietary restriction should be continued and monitored indefinitely..# Pregnancy risks can be minimized by continuing lifelong phenylalanine-restricted diets and assuring strict phenylalanine restriction 2 months prior to conception and throughout gestation.ConditionEnzyme DefectClinical FindingsInheritance1. Phenylketonuria type IPhenylalanine hydroxylaseMental retardation, microcephaly, hypopig- mented skin and hairs, eczema, "mousy" odorAR2. Phenylketonuria type IIDihydropteridine reductaseMental retardation, hypotonia, spasticity, myoclonusAR3. Phenylketonuria type III6-Pyruvoyl- tetrahydropterin synthaseDystonia, neurologic deterioration, seizures, mental retardationAR4. GTP cyclohydrolase I deficiencyGTP cyclohydrolase IMental retardation, seizures, dystonia, temperature instabilityAR5. Carbinolamine dehydratase deficiencyPterin-4-carbinolamine dehydrataseTransient hyperphenylalaninemia (benign)AR">