Definition check: what makes an amino acid “essential” in human nutrition? Select the most accurate definition used in biochemistry and dietetics.

Introduction / Context:
Dietary planning and clinical nutrition rely on the distinction between essential and nonessential amino acids. Essential amino acids must be provided in adequate amounts through food.

Given Data / Assumptions:

• Applies to humans (species dependence exists).
• Focus is on biosynthetic capability under normal physiology.

Concept / Approach:
Essential amino acids are those humans cannot synthesize de novo in adequate amounts. They must be ingested. Examples include leucine, isoleucine, valine, lysine, threonine, tryptophan, phenylalanine, methionine, and in some contexts histidine (and arginine conditionally for growth).

Step-by-Step Solution:
Define “essential” as dietary requirement due to absent or insufficient biosynthesis.Eliminate options that refer to ease of synthesis or unrelated cellular functions.Select the definition stating the body cannot synthesize.

Verification / Alternative check:
Clinical formulas and RDAs reflect this requirement, ensuring these amino acids are included.

Why Other Options Are Wrong:
Flagellar motion is unrelated to nutritional classification; “under essential conditions” is vague and incorrect.

Common Pitfalls:
Confusing “conditionally essential” (for example, during growth or illness) with universally essential amino acids.

Final Answer:
The body cannot synthesize.