**Core Concept**
Selenocysteine is a rare amino acid found in some proteins, playing a crucial role in the functioning of enzymes involved in antioxidant defenses, such as glutathione peroxidase. It is incorporated into proteins through a unique mechanism involving the incorporation of selenide into a cysteine residue.

**Why the Correct Answer is Right**
The correct answer is related to the biosynthesis of selenocysteine, which begins with the formation of selenophosphate from ATP and selenide. This intermediate is then used to form selenocysteine by the enzyme selenocysteine synthase, which is similar to cysteine synthase. The selenocysteine is then incorporated into proteins in place of cysteine, where it forms a selenol group that is essential for the enzymatic activity of glutathione peroxidase.

**Why Each Wrong Option is Incorrect**
**Option A:** Methionine - Methionine is an amino acid with a sulfur-containing side chain, but it is not directly involved in the biosynthesis of selenocysteine.
**Option B:** Cysteine - While cysteine is similar to selenocysteine, it is not the precursor to selenocysteine, but rather the amino acid from which selenocysteine is derived.
**Option C:** Tyrosine - Tyrosine is an amino acid with a phenolic side chain and is not involved in the biosynthesis of selenocysteine.

**Clinical Pearl / High-Yield Fact**
Selenocysteine is an essential component of glutathione peroxidase, an enzyme that helps protect cells from oxidative damage by reducing lipid hydroperoxides. This enzyme is particularly important in the liver, where it helps to detoxify harmful substances.

**Correct Answer:** B. Cysteine