**Core Concept:** Glutathione is an essential antioxidant containing three amino acids: glycine, glutamic acid, and cysteine. Glutathione is vital for maintaining cellular redox balance and is involved in various physiological processes.

**Why the Correct Answer is Right:** The correct answer, **cysteine**, is the amino acid that acts as the reducing agent in glutathione. The presence of a sulfhydryl group (-SH) in cysteine enables its ability to donate electrons and participate in redox reactions, which is essential for glutathione's antioxidant function.

**Why Each Wrong Option is Incorrect:**

A. **Glycine:** While glycine is one of the components of glutathione, it does not have a sulfhydryl group (-SH) that is essential for its reducing properties.

B. **Glutamic acid:** Glutamic acid is also a component of glutathione but does not possess a sulfhydryl group (-SH), making it unable to act as a reducing agent.

C. **Alanine:** Alanine is an amino acid found in glutathione but lacks the sulfhydryl group (-SH), preventing it from functioning as a reducing agent.

**Clinical Pearl / High-Yield Fact:**
Understanding the role of amino acids in glutathione is crucial in understanding cellular antioxidant defense, drug interactions, and the implications of glutathione depletion in various diseases, such as cystic fibrosis and liver disorders.

**Correct Answer:** **cysteine** is the correct answer because it contains a sulfhydryl group (-SH) enabling its role as a reducing agent in glutathione.