Which substance cannot be converted into tyrosine in individuals with PKU?

In individuals with phenylketonuria (PKU), the enzyme phenylalanine hydroxylase is deficient or absent, leading to an inability to convert phenylalanine into tyrosine. This is critical because tyrosine is an important amino acid that is precursory to neurotransmitters such as dopamine, norepinephrine, and epinephrine. Individuals with PKU must follow a strict diet to limit phenylalanine intake, as elevated levels can cause harmful effects, including intellectual disability and other neurological issues.

The other substances listed—cysteine, arginine, and glycine—play various roles in metabolism but are not directly involved in the conversion process to tyrosine in individuals with PKU. While cysteine can be synthesized from methionine and is part of a different metabolic pathway, it will not serve as a substitute for phenylalanine in the synthesis of tyrosine. Similarly, arginine and glycine are also not able to be converted into tyrosine, highlighting that phenylalanine is specifically the amino acid that needs to be converted for making tyrosine in a normal metabolic context. Thus, individuals with PKU face issues specifically with phenylalanine, not with