Multiple choice Phenylketonuria is caused by a defect in the gene for an enzyme that converts: Tyrosine to phenylalanine causing a buildup of phenylalanine Phenylalanine to tyrosine causing a buildup of phenylalanine Phenylalanine to tyrosine causing a buildup of tyrosine Tyrosine to phenylalanine causing a buildup of tyrosine Reveal answer Fill a bubble to check yourself B Correct answer Explanation PKU is caused by a deficiency of the enzyme phenylalanine hydroxylase. This enzyme is responsible for converting the amino acid phenylalanine into tyrosine.