## Core Concept
Glutathione is a tripeptide antioxidant produced in cells, composed of cysteine, glutamic acid, and glycine. It plays a crucial role in maintaining cellular redox balance and protecting against oxidative stress. Glutathione exists in two forms: reduced glutathione (GSH) and oxidized glutathione (GSSG).

## Why the Correct Answer is Right
The correct answer, , highlights that glutathione acts as a major antioxidant in cells. This is accurate because glutathione's thiol group (-SH) donates electrons to reduce reactive oxygen species (ROS) and detoxify harmful substances. The enzyme glutathione peroxidase utilizes GSH to neutralize hydrogen peroxide and lipid hydroperoxides, converting GSH to GSSG in the process. GSSG is then reduced back to GSH by glutathione reductase, maintaining the cellular redox balance.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because, although glutathione does play a role in detoxification processes, stating it is primarily involved in detoxification overlooks its critical antioxidant functions.
- **Option B:** This option is incorrect as it inaccurately describes glutathione's composition or function. Without the specific text of option B, we can infer that any statement contradicting glutathione's established roles or characteristics would be incorrect.
- **Option C:** Similarly, without the specific details of option C, any statement not aligning with glutathione's antioxidant properties, detoxification roles, or its structure as a tripeptide would be incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **N-acetylcysteine (NAC)**, an amino acid, serves as a precursor to cysteine, a component of glutathione. NAC is used clinically to replenish glutathione stores, particularly in conditions like acetaminophen overdose, which depletes glutathione.

## Correct Answer Line
**Correct Answer: D. Glutathione acts as a major antioxidant in cells.**