**Question:** The amino acid residue having an imino side chain is:

A. Arginine
B. Cysteine
C. Glutamine
D. Histidine

**Core Concept:**
The question is asking about amino acids with an imino side chain, which includes a nitrogen atom bonded to the carbon atom of the amide group. There are four amino acids that fit this description:

**Why the Correct Answer is Right:**
Arginine (A) is a non-essential amino acid that has an imino side chain. It is synthesized in the body and is involved in various physiological processes, such as nitric oxide production, cation transport, and protein synthesis.

**Why Each Wrong Option is Incorrect:**

B. Cysteine (B) has a thiol side chain, which is different from an imino side chain. It is an essential amino acid and plays a role in protein synthesis, redox reactions, and disulfide bond formation.

C. Glutamine (C) has an amide side chain, which is different from an imino side chain. It is an essential amino acid and plays a crucial role in protein synthesis, neurotransmission, and energy production.

D. Histidine (D) has an imidazole side chain, which is different from an imino side chain. It is an essential amino acid and participates in protein synthesis, enzymatic reactions, and the formation of heme.

**Clinical Pearl:**
Understanding the distinct side chains of different amino acids is crucial for understanding protein structure, function, and regulation. Imino acids and amide acids have distinct roles in various physiological processes and are essential components of proteins, enzymes, and signaling molecules in the human body.