**Core Concept**
Glutathione peroxidase is an enzyme that plays a crucial role in protecting cells from oxidative damage by reducing lipid hydroperoxides and hydrogen peroxide. This enzyme is a key component of the antioxidant defense system in cells.

**Why the Correct Answer is Right**
Glutathione peroxidase contains selenocysteine (Sec), a unique amino acid that is essential for its catalytic activity. Selenocysteine is incorporated into the enzyme through a process known as selenoprotein synthesis, which involves the translation of a specific codon (UGA) as selenocysteine instead of the usual stop codon. The selenocysteine residue in glutathione peroxidase is responsible for the enzyme's ability to reduce hydrogen peroxide and lipid hydroperoxides, thus protecting cells from oxidative damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Copper (Cu) is not a component of glutathione peroxidase, although it is a key component of other antioxidant enzymes such as superoxide dismutase.
**Option C:** Iron (Fe) is not a component of glutathione peroxidase, although it is a key component of other enzymes involved in oxidative stress, such as catalase.
**Option D:** Mercury (Hg) is not a component of glutathione peroxidase and is actually toxic to cells at high concentrations.

**Clinical Pearl / High-Yield Fact**
Glutathione peroxidase is a crucial enzyme in the antioxidant defense system, and its activity is often impaired in conditions such as cancer, atherosclerosis, and neurodegenerative diseases. Supplements containing selenium, the essential trace element required for glutathione peroxidase activity, may have antioxidant effects and help to prevent oxidative stress.

**✓ Correct Answer: B. Se. Selenocysteine is essential for glutathione peroxidase activity.**