Which of the following amino acids is not formed in the person suffering from phenylketonuria?

Persons with phenylketonuria do not form the enzyme phenylalanine hydroxylase and hence phenylalanine can not be converted to tyrosine. Phenylalanine may be converted to a ketone known as phenylpyruvic acid which is accumulated in cerebral tissue along with phenylalanine and causes mental retardness. Loss of skin pigmentation is also observed as tyrosine is required for the formation of melanin.