Phenylketonuria is caused by the accumulation of which substance?

The correct answer is Option (2) → Phenylpyruvic acid

Phenylketonuria is an inborn error of metabolism, inherited as an autosomal recessive trait. The affected person lacks the enzyme that changes the amino acid phenylalanine into tyrosine. So phenylalanine builds up and is converted into phenylpyruvic acid and other derivatives. These collect in the brain and cause mental retardation. Since the kidney absorbs them poorly, they also pass out in the urine.

NCERT Extract:
Mendelian Disorders – “… Phenylketonuria : This inborn error of metabolism is also inherited as the autosomal recessive trait. The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine. As a result of this phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives. Accumulation of these in brain results in mental retardation. These are also excreted through urine because of its poor absorption by kidney.”