**Core Concept**
At physiological pH (around 7.4), amino acids can exist in different ionized forms. The stability of an amino acid is determined by its pKa values and the pH of the environment. Amino acids with pKa values near physiological pH are more likely to be in a charged state, affecting their stability.

**Why the Correct Answer is Right**
Histidine is the most stable amino acid at physiological pH because its imidazole side chain has a pKa value close to 7, allowing it to be in a zwitterionic state (both positively and negatively charged) at this pH. This unique property makes histidine more resistant to denaturation and degradation compared to other amino acids. The presence of an imidazole ring in histidine also enables it to participate in various biochemical reactions, such as enzyme catalysis.

**Why Each Wrong Option is Incorrect**
**Option B:** Lysine has a basic side chain with a pKa value around 10.5, making it highly charged at physiological pH and thus less stable than histidine.
**Option C:** Arginine has a basic side chain with a pKa value around 12, making it even more charged than lysine and highly unstable at physiological pH.
**Option D:** Glycine is the simplest amino acid and lacks a charged side chain, making it relatively unstable at physiological pH compared to histidine.

**Clinical Pearl / High-Yield Fact**
Histidine is an essential amino acid, meaning it cannot be synthesized by the human body and must be obtained through the diet. It plays a crucial role in enzyme function, particularly in the synthesis of histamine, a key mediator in allergic reactions and immune responses.

**✓ Correct Answer: A. Histidine**