Classical PKU is caused by a deficiency of the liver enzyme phenylalanine hydroxylase, which converts the amino acid phenylalanine to tyrosine.
The classic example of this is in the disorder called phenylketonuria (PKU ).
However, it is important to note that infants suffering from the rare genetic disease phenylketonuria (PKU) should not consume aspartame.
Single gene defects such as phenylketonuria (PKU) and other inborn errors of metabolism may also cause mental retardation if they are not found and treated early.
Conditions such as hyperthyroidism and PKU are treatable.