Which amino acid is essential and must be obtained from the diet?

• A. Arginine
• B. Lysine
• C. Methionine
• D. Leucine

Answer: (B)

Essential amino acids are those the body cannot make in adequate amounts, so they must come from the diet. Lysine is one of these essential amino acids, meaning the body cannot produce enough of it on its own to support normal protein synthesis and metabolism, so dietary intake is necessary. Arginine, by contrast, is considered conditionally essential—most people can synthesize enough under normal conditions, though there are times (like rapid growth or illness) when it may become needed from food. Leucine and methionine are also essential amino acids, but the question points to lysine as the example of an amino acid that must be supplied by dietary protein.