What compound accumulates in the blood of people who have PKU?

Phenylalanine

Explanation:

Phenylketonuria is an inherited genetic disorder caused by the build-up of the amino acid, phenylalanine. PKU is autosomal recessive. This means that an individual must possess two copies of the defective gene, one from each parent, to be affected by PKU.

Nerve cells are extremely sensitive to phenylalanine amounts. Therefore, accumulation of extremely high amounts of phenylalanine causes neurological disabilities, particularly, in children where it can cause permanent brain damage, seizures and delayed growth.

Phenylketonuria is caused by a mutation in the PAH gene that encodes for an enzyme, phenylalanine hydroxylase that converts phenylalanine to another amino acid, tyrosine.