**Core Concept**
Histidine is a unique amino acid due to the presence of an imidazole side chain, which plays a crucial role in its physiological activity. At neutral pH (around 7.4), the imidazole group of histidine is protonated, giving it a positive charge. This characteristic allows histidine to participate in various biochemical processes, including enzyme catalysis and protein function.

**Why the Correct Answer is Right**
Histidine's imidazole group is essential for its activity at neutral pH. The imidazole ring contains a nitrogen atom that can accept a proton (H+), making it a good catalyst for various biochemical reactions. This property is particularly useful in enzymes, such as histidine-containing carbonic anhydrase, where the imidazole group facilitates the transfer of protons. The unique pKa of histidine's imidazole group (around 6.0) allows it to maintain a positive charge at neutral pH, making it an ideal candidate for biological reactions.

**Why Each Wrong Option is Incorrect**
**Option B:** Glycine is the simplest amino acid, with only a hydrogen atom as its side chain. Its lack of a functional group or a side chain makes it inactive at neutral pH.
**Option C:** Leucine is a non-polar, hydrophobic amino acid with a large, non-reactive side chain. Its neutral side chain does not contribute to its activity at neutral pH.
**Option D:** Arginine is a positively charged amino acid at physiological pH due to its guanidinium group. However, its positive charge is not due to the same imidazole group mechanism as histidine, making it less relevant to the question.

**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. A deficiency in histidine can lead to impaired growth and development, particularly in children.

**✓ Correct Answer: A. Histidine**