Phenylketonuria is an inherited genetic disorder caused by a deficiency of the enzyme phenylalanine hydroxylase. This enzyme is responsible for converting the amino acid phenylalanine into another amino acid, tyrosine. Due to the lack of this enzyme, phenylalanine accumulates and is converted into phenylpyruvic acid, which can have toxic effects on the body. As a result, this disorder is also known as PKU or Phenylketonuria.