Give scientific reasons: The product of phenylalanine catabolism excreted in the urine in phenylketonuria.

(N/A) Phenylketonuria $(PKU)$ is an inborn error of metabolism caused by the deficiency of the enzyme $Phenylalanine$ hydroxylase.
This enzyme is essential for converting the amino acid $Phenylalanine$ into $Tyrosine$ in the liver.
Due to the lack of this enzyme, $Phenylalanine$ accumulates in the body and is diverted into an alternative metabolic pathway.
It gets converted into $Phenylpyruvic$ acid and other related derivatives.
These metabolites accumulate in the brain, leading to mental retardation, and are also excreted in the urine due to poor reabsorption by the kidneys.