**Core Concept**
Sulphur-containing amino acids are essential components of proteins, playing crucial roles in various biochemical processes, including enzyme function, antioxidant defenses, and protein structure. These amino acids contain a sulphur atom in their side chain, which is vital for their biological activity.

**Why the Correct Answer is Right**
The correct answer is the amino acid that does not contain a sulphur atom in its structure. The sulphur-containing amino acids found in proteins are methionine and cysteine. Methionine serves as a precursor for the synthesis of other sulphur-containing compounds, while cysteine is essential for the formation of disulfide bridges, which stabilize protein structures. The absence of a sulphur atom in the structure of the correct answer makes it distinct from the other options.

**Why Each Wrong Option is Incorrect**
**Option A:** Methionine is a sulphur-containing amino acid, making it incorrect.
**Option B:** Cysteine is another sulphur-containing amino acid, making it incorrect.
**Option D:** Tryptophan is an aromatic amino acid but does not contain a sulphur atom in its structure.

**Clinical Pearl / High-Yield Fact**
Sulphur-containing amino acids are essential for maintaining proper protein structure and function. Deficiencies in these amino acids can lead to various clinical manifestations, including impaired enzyme activity and compromised immune function.

**Correct Answer: D. Tryptophan.**