## **Core Concept**
Amino acids, the building blocks of proteins, can exist in various forms, including optically active and inactive forms. Optical activity in amino acids is primarily due to the presence of a chiral center (asymmetric carbon atom), which allows them to exist as D- or L- enantiomers. However, some amino acids lack this chiral center or have a structure that makes them optically inactive.

## **Why the Correct Answer is Right**
The correct answer, **Glycine**, is optically inactive because it has two hydrogen atoms attached to its alpha carbon, making it the only amino acid without a chiral center. This structural characteristic means glycine does not have D- or L- enantiomers and thus is not optically active.

## **Why Each Wrong Option is Incorrect**
- **Option A: Leucine** - Leucine has a chiral center and exists as L- and D- forms, making it optically active.
- **Option B: Isoleucine** - Similar to leucine, isoleucine also has a chiral center and can exist as different enantiomers, making it optically active.
- **Option D: Threonine** - Threonine has two chiral centers, which allows it to exist in different diastereomeric forms, and it is optically active.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while glycine is optically inactive due to its lack of a chiral center, this characteristic does not diminish its importance in protein structure and function. Glycine's small size and flexibility allow it to play critical roles in protein folding and in the formation of turns and loops in proteins.

## **Correct Answer: . Glycine**