**Core Concept**
Essential amino acids are those that cannot be synthesized by the human body and must be obtained from the diet. These are nine in number and include leucine, lysine, methionine, and others. Proline is not considered essential because it can be synthesized from glutamate via the enzyme glutamate-γ-oxaloacetate transaminase (GOT) and further metabolism.

**Why the Correct Answer is Right**
Proline is a conditionally essential amino acid under certain conditions (e.g., severe illness), but it is not classified as essential because the body can synthesize it from glutamate through the pathway involving glutamate dehydrogenase and ornithine. In contrast, leucine, lysine, and methionine are strictly essential and cannot be produced endogenously. Thus, proline is the exception among the listed amino acids.

**Why Each Wrong Option is Incorrect**
Option A: Leucine is one of the nine essential amino acids and cannot be synthesized by humans.
Option B: Lysine is essential and required in the diet; it plays a key role in protein synthesis and collagen formation.
Option C: Methionine is essential and serves as a precursor for cysteine and other sulfur-containing compounds.

**Clinical Pearl / High-Yield Fact**
Remember the "Essential Amino Acids" list: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine. Proline is a non-essential amino acid and is synthesized in the body — a key point for biochemistry and nutrition exams.

✓ Correct Answer: D. Proline