**Core Concept**
Glutathione is a tripeptide antioxidant composed of cysteine, glutamic acid, and glycine. It plays a crucial role in maintaining cellular redox balance and detoxifying harmful substances.

**Why the Correct Answer is Right**
Glutathione's antioxidant properties are primarily attributed to the sulfhydryl group (-SH) of cysteine, which can donate electrons to reduce oxidized molecules. This process is mediated by glutathione reductase, an enzyme that maintains the reduced form of glutathione (GSH). GSH is also involved in the conjugation and excretion of toxins, such as heavy metals and xenobiotics.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because glutathione is not a pro-oxidant; it is an antioxidant that helps reduce oxidative stress.
**Option B:** Incorrect because glutathione peroxidase is an enzyme that requires glutathione as a cofactor, but it is not a component of glutathione itself.
**Option C:** Incorrect because glutathione does not have a direct role in the citric acid cycle; it is primarily involved in antioxidant defense mechanisms.

**Clinical Pearl / High-Yield Fact**
Glutathione levels are often depleted in conditions of oxidative stress, such as sepsis, trauma, and cancer. Supplementing with glutathione or its precursors may be beneficial in these situations.

**Correct Answer: None**