**Core Concept**
Glutathione is a tripeptide antioxidant composed of amino acids that plays a crucial role in cellular defense mechanisms, detoxification processes, and maintaining cellular redox balance. It is primarily synthesized in the liver and is involved in various biochemical reactions, including enzyme activation and antioxidant defense.

**Why the Correct Answer is Right**
Glutathione is composed of three amino acids: glutamic acid, cysteine, and glycine. The cysteine residue in glutathione contains a thiol group (-SH), which is responsible for its antioxidant properties. The thiol group can donate electrons to reactive oxygen species, neutralizing them and preventing oxidative damage to cellular components. The glutamic acid and glycine residues in glutathione also play important roles in its synthesis and function.

**Why Each Wrong Option is Incorrect**
**Option A:** Glutathione is not composed of alanine, aspartic acid, and glutamic acid. While glutamic acid is indeed a component of glutathione, the other two amino acids listed are not part of its structure.
**Option B:** Glutathione is not composed of serine, threonine, and proline. These amino acids are not involved in the synthesis or structure of glutathione.
**Option C:** Glutathione is not composed of valine, leucine, and isoleucine. These branched-chain amino acids are not part of the glutathione molecule.

**Clinical Pearl / High-Yield Fact**
Glutathione is often depleted in patients with chronic diseases, such as liver cirrhosis, cancer, and HIV/AIDS, and supplementation may be beneficial in these conditions. Additionally, glutathione levels can be measured in blood or urine to assess oxidative stress and antioxidant defense.

**Correct Answer:** D. Glutamic acid, cysteine, and glycine.