## **Core Concept**
The question tests knowledge of amino acid structure, specifically identifying which amino acid's side chain contains a sulfhydryl group. Amino acids are the building blocks of proteins, and their side chains confer unique chemical properties. The sulfhydryl group, also known as a thiol, is a functional group that contains a sulfur-hydrogen bond.

## **Why the Correct Answer is Right**
The correct answer, Cysteine, has a side chain that contains a sulfhydryl group (-SH). This group is crucial for the chemical properties of cysteine, including its ability to form disulfide bonds with other cysteine residues, which is important for protein structure and function. The sulfhydryl group is highly reactive and can participate in various biochemical reactions.

## **Why Each Wrong Option is Incorrect**
- **Option A: Glycine** is the simplest amino acid with a side chain that consists of just a hydrogen atom. It does not contain a sulfhydryl group.
- **Option B: Serine** has a hydroxyl group (-OH) in its side chain, not a sulfhydryl group. It is polar and uncharged at physiological pH.
- **Option D: Tyrosine** contains a phenolic hydroxyl group in its side chain, which is different from a sulfhydryl group. Tyrosine is aromatic and can be phosphorylated in proteins.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that cysteine's sulfhydryl group is essential for the antioxidant properties of glutathione, a tripeptide that includes cysteine. Glutathione plays a critical role in protecting cells from oxidative damage. This highlights the importance of cysteine in biochemistry and medicine.

## **Correct Answer:** C. Cysteine