**Core Concept**
Thyroxine, a hormone produced by the thyroid gland, is synthesized from an amino acid through a series of biochemical reactions. The process involves the incorporation of iodine into the amino acid, which is then used to form the thyroxine molecule.

**Why the Correct Answer is Right**
Tyrosine is the amino acid required for the formation of thyroxine. This is because the enzyme thyroid peroxidase (TPO) catalyzes the iodination of tyrosine residues on thyroglobulin, a protein produced by the thyroid gland. The iodinated tyrosine residues are then coupled together to form the thyroxine (T4) molecule. Tyrosine is the only amino acid that can be iodinated and used for thyroid hormone synthesis.

**Why Each Wrong Option is Incorrect**
**Option A:** Tryptophan is an essential amino acid involved in the synthesis of serotonin, melatonin, and niacin, but it is not required for thyroid hormone synthesis.
**Option C:** Glutamine is a non-essential amino acid that serves as a nitrogen donor in various biochemical reactions, but it is not involved in the synthesis of thyroid hormones.
**Option D:** Cysteine is an essential amino acid that is involved in the synthesis of glutathione, but it is not required for the formation of thyroid hormones.

**Clinical Pearl / High-Yield Fact**
It's worth noting that iodine deficiency can lead to reduced synthesis of thyroid hormones, including thyroxine. This can result in hypothyroidism, a condition characterized by decreased metabolic rate, fatigue, and dry skin.

**✓ Correct Answer: B. Tyrosine**