**Core Concept:**
Understanding the polarity of side chains in amino acids is crucial for understanding protein structure, function, and interactions with other molecules. Amino acids are classified into two groups based on their side chain polarity:

1. Aliphatic (non-polar) amino acids: These have non-polar side chains, which can be further divided into:
- Hydrophobic: Uncharged, non-polar side chains that repel water.
- Aromatic: Polar but contain a benzene ring, which makes them less polar than other polar groups.

2. Polar (charged or hydrogen-bearing) amino acids: These have polar side chains, including:
- Charged amino acids: Positively charged (Lysine, Arginine) or negatively charged (Glutamate, Aspartate).
- Hydrogen-bearing amino acids: Glycine, Serine, Threonine, and Proline.

**Why the Correct Answer is Right:**
The least polar side chain among the given options belongs to the aromatic amino acid, Phenylalanine. Aromatic amino acids have a benzene ring in their side chains, which gives them a lower polarity compared to other polar groups but higher than non-aromatic, non-polar side chains.

**Why Each Wrong Option is Incorrect:**
A. Tyrosine: This amino acid has a hydroxyl (-OH) group in its side chain, making it more polar than Phenylalanine.
B. Histidine: Although Histidine has a histidine ring, its side chain also contains a polar imidazole group, making it more polar than Phenylalanine.
C. Isoleucine: This amino acid has a non-polar side chain, but it is not the least polar among the options.
D. Leucine: Similar to Isoleucine, this amino acid has a non-polar side chain but is not the least polar.

**Clinical Pearl:**
Understanding amino acid side chains and their polarity is crucial for understanding protein structure, folding, and interactions with other molecules. Polarity plays a significant role in determining protein solubility, stability, and interactions with other biomolecules, such as lipids or water. Aromatic amino acids, like Phenylalanine, are involved in the synthesis of neurotransmitters and hormones, such as dopamine, norepinephrine, and serotonin.

**Correct Answer Explanation:**
The correct answer is Phenylalanine (Option D). Phenylalanine has an aromatic side chain, which is a benzene ring, making it less polar than the other amino acids mentioned. The other options are incorrect due to having hydroxyl (-OH) groups (Tyrosine, Histidine) or being non-polar (Isoleucine, Leucine).