Proteins

To determine which amino acid among the given options is non-essential, we need to understand the difference between essential and non-essential amino acids.

Essential Amino Acids: These cannot be synthesized by the body and must be obtained from the diet. Examples include valine, leucine, and lysine.

Non-Essential Amino Acids: These can be synthesized by the body, making it unnecessary to obtain them through diet. An example is alanine.

Let's evaluate each option:

1. Valine: Valine is an essential amino acid, which means it cannot be produced by the body and must be taken in through food.
2. Leucine: Like valine, leucine is also an essential amino acid, required to be consumed through dietary proteins.
3. Alanine: Alanine is a non-essential amino acid. The body can synthesize it, and therefore it doesn't need to be obtained from an external food source.
4. Lysine: Lysine is another essential amino acid, necessary for growth and tissue repair, and must be obtained through the diet.

From the evaluation, Alanine is identified as the non-essential amino acid, as it can be synthesized by the body. Hence, the correct answer is alanine.