**Core Concept**
Transamination is a biochemical reaction that involves the transfer of an amino group from an amino acid to a keto acid, resulting in the formation of a new amino acid and a new keto acid. This reaction is crucial in the synthesis and degradation of amino acids and plays a significant role in nitrogen metabolism.

**Why the Correct Answer is Right**
Transamination reactions require the presence of a coenzyme called pyridoxal phosphate (PLP), which is derived from vitamin B6. PLP acts as a coenzyme for the transaminase enzymes, facilitating the transfer of the amino group from the amino acid to the keto acid. This reaction is catalyzed by transaminase enzymes, which are specific to each amino acid. The correct answer is related to the coenzyme required for transamination reactions.

**Why Each Wrong Option is Incorrect**
**Option A:** Glutathione is a tripeptide involved in antioxidant defenses and detoxification reactions, but it is not directly involved in transamination reactions.
**Option B:** FADH2 is a coenzyme involved in the electron transport chain and fatty acid oxidation, but it is not required for transamination reactions.
**Option C:** UDP-glucose is a coenzyme involved in glycogen synthesis and glycolysis, but it is not related to transamination reactions.

**Clinical Pearl / High-Yield Fact**
Transamination reactions are essential for nitrogen metabolism and play a critical role in the synthesis and degradation of amino acids. Understanding the coenzyme requirements for transamination reactions is crucial for diagnosing and managing disorders related to amino acid metabolism.

**Correct Answer: D. Pyridoxal phosphate (PLP)**