**Core Concept**
Glutathione is a tripeptide composed of glutamic acid, cysteine, and glycine. It plays a crucial role in cellular defense against oxidative stress by scavenging reactive oxygen species (ROS) and maintaining the balance of redox reactions.

**Why the Correct Answer is Right**
The reducing agent in glutathione is cysteine. The thiol group (-SH) in cysteine is responsible for its reducing properties. When glutathione is oxidized, the thiol group in cysteine is converted to a disulfide bond, which can then be reduced back to its original state by NADPH-dependent enzymes. This cycle of oxidation and reduction helps maintain the cellular redox balance. The cysteine residue in glutathione can donate electrons to oxidized molecules, reducing them back to their original state.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glutamic acid is not the reducing agent in glutathione. While it is a component of the tripeptide, its primary function is to provide a carboxyl group for electrostatic interactions.
* **Option B:** Glycine is the smallest amino acid and serves as a structural component of glutathione. However, it does not possess reducing properties.
* **Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
Glutathione is a critical antioxidant in the body, and its levels can be depleted in conditions of oxidative stress, such as sepsis, trauma, and cancer. Maintaining glutathione levels is essential for protecting against cellular damage and promoting tissue repair.

**Correct Answer:** C. Cysteine.