Final answer:
Amino acids that become essential due to specific medical conditions or life stages are known as conditionally essential amino acids, such as in the case of PKU where tyrosine becomes conditionally essential.
Step-by-step explanation:
Amino acids that are only essential due to medical conditions or lifespan periods are termed conditionally essential amino acids. The body cannot synthesize nine amino acids, making them essential amino acids, and these must be consumed through diet. However, there are certain situations, such as medical conditions like phenylketonuria (PKU), in which non-essential amino acids become conditionally essential. People with PKU have trouble converting phenylalanine to tyrosine, making the latter a conditionally essential amino acid for them.