**Core Concept**
Thyroid hormone formation begins with the synthesis of thyroxine (T4) and triiodothyronine (T3) from tyrosine residues on thyroglobulin, a large protein produced by thyroid follicular cells. This process involves iodination, coupling, and oxidation reactions.

**Why the Correct Answer is Right**
Tyrosine is the amino acid precursor for thyroxine because it contains a phenolic group, which is essential for iodination. Thyroid peroxidase (TPO) catalyzes the iodination of tyrosine residues on thyroglobulin, followed by coupling of two iodinated tyrosine residues to form diiodotyrosine (DIT). Two DIT molecules are then coupled to form thyroxine (T4). This process is crucial for the production of thyroid hormones in the thyroid gland.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycine is not directly involved in thyroid hormone synthesis, although it may be present in the thyroglobulin protein backbone.
**Option C:** Phenylalanine is an essential amino acid involved in various biochemical pathways, but it is not a direct precursor for thyroxine.
**Option D:** Iodine is essential for thyroid hormone synthesis, but it is not an amino acid and does not serve as a precursor for thyroxine.

**Clinical Pearl / High-Yield Fact**
Remember that tyrosine is a key starting material for thyroid hormone synthesis, and defects in this pathway can lead to congenital hypothyroidism, a condition where the thyroid gland is unable to produce sufficient thyroid hormones.

✓ Correct Answer: B. Tyrosine