Final answer:
Tyrosine is an example of a conditionally indispensable amino acid, becoming essential in the circumstances of phenylketonuria (PKU) due to a mutation in phenylalanine hydroxylase.
Step-by-step explanation:
An example of a conditionally indispensable amino acid is B. Tyrosine. Conditionally indispensable, or conditionally essential amino acids become essential under certain circumstances, such as in the presence of certain diseases. For instance, in individuals with phenylketonuria (PKU), the enzyme phenylalanine hydroxylase is mutated and cannot convert phenylalanine to tyrosine efficiently. As a result, tyrosine becomes an essential amino acid for these individuals because their bodies cannot produce enough of it on their own.
Non-essential amino acids, including tyrosine, are not required in the diet for most individuals because the body can usually synthesize them. However, because individuals with PKU cannot typically make enough tyrosine due to their condition, it becomes essential for them to obtain it through their diet or supplementation.