phenylketonuria

n. (PKU) The presence of high levels of phenylketones (phenylpyruvic acid) in the urine due to an inherited inability to metabolize phenylalanine, which is caused by the absence of phenylalanine hydroxylase and leads to high levels of phenylalanine in the blood, which, in turn, cause severe mental retardation unless diet is controlled to prevent the ingestion of foods containing phenylalanine.