People Who Have Phenylketonuria, a Genetic Disorder, Are Unable to Break

People who have phenylketonuria, a genetic disorder, are unable to break down the amino acid phenylalanine so that it can be used in cellular respiration. Yet they need phenylalanine in their diet. Why do they need to eat some foods with phenylalanine?

A) Amino acids yield the most energy per gram, so they need phenylalanine to supply their cells with energy.
B) Amino acids are used to build proteins and so they need enough phenylalanine for biosynthesis without any leftover that needs to be broken down to enter cellular respiration.
C) Cellular respiration can use proteins more readily than other types of molecules, so the amino acid phenylalanine is essential.