In phenylketonuria :-

Explanation

It is a rare disease in which individual lacks an enzyme called Phenylalanine hydroxylase. Which is needed to breakdown an essential amino acid phenylalanine into tyrosine in liver. This phenylalanine is accumulated and gets converted into phenylpyruvic acid and other derivatives leading to mental retardation (due to accumulation in brain). These are also excreted through urine because of its poor absorption by kidney.