Which of the following is not a non-essential amino acid?

Lysine is considered an essential amino acid, unlike alanine, glutamine, and serine, which are classified as non-essential. Essential amino acids are those that cannot be synthesized by the body and must be obtained through diet, while non-essential amino acids can be produced by the body even if they are not consumed in the diet.

Alanine, glutamine, and serine play various roles in the body, such as energy production and neurotransmitter functions, making them important for overall health, but they do not need to be ingested directly since the body can create them. This distinction of lysine as an essential amino acid highlights its requirement in the diet, while the others can be synthesized as needed.