Final answer:
In phenylketonuria (PKU), the conditionally essential amino acid is tyrosine.
Step-by-step explanation:
In phenylketonuria (PKU), the amino acid that becomes conditionally essential is tyrosine. People with PKU have a mutation in the enzyme phenylalanine hydroxylase, which results in the inability to properly break down phenylalanine into tyrosine.
As a result, phenylalanine levels rise to toxic levels, causing damage to the central nervous system and brain. To manage PKU, individuals must follow a strict diet low in phenylalanine and avoid foods high in phenylalanine, such as artificial sweeteners like aspartame.