Medical Definition of phenylketonuria
: a metabolic disorder that is caused by an enzyme deficiency resulting in the accumulation of phenylalanine and its metabolites (as phenylpyruvic acid) in the blood and their excess excretion in the urine, that is inherited as an autosomal recessive trait, and that causes usually severe mental retardation, seizures, eczema, and abnormal body odor unless phenylalanine is restricted from the diet beginning at birth—abbreviation PKU
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