Final answer:
The biochemical defect in phenylketonuria (PKU) is a functional deficiency of the liver enzyme phenylalanine hydroxylase, which catalyzes the para-hydroxylation of phenylalanine to yield tyrosine.
Step-by-step explanation:
The biochemical defect in phenylketonuria (PKU) is a functional deficiency of the liver enzyme phenylalanine hydroxylase, which catalyzes the para-hydroxylation of phenylalanine to yield the amino acid tyrosine.