noun. an hereditary metabolic illness transmitted as an autosomal recessive trait and characterized by an insufficiency of an enzyme required to use the amino acid phenylalanine. Unless it is identified in formative infancy and remedied by a limited dietary intake of phenylalanine, phenylketonuria generates intense cognitive retardation and other nervous-system illnesses and dysfunctions.

PHENYLKETONURIA (PKU): "The child has been diagnosed with phenylketonuria."