PKU is the abbreviation of phenylketonuria, and it is caused by a deficiency of phenylalanine hydroxylase or more rarely from a deficiency of tetrahydrobiopterin, which is required for the phe to tyr conversion. As a result of this deficiency, phenylalanine cannot be converted to tyrosine. Phenylalanine accumulates in the tissues and is transaminated to phenylpyruvate which is elevated in the blood and excreted in the urine, hence the name. The structure of phenylpyruvate is