**Core Concept**
Melanin is a pigment synthesized in melanocytes through a series of enzymatic reactions in the tyrosine metabolism pathway. The initial substrate for melanin production is the amino acid tyrosine, which undergoes hydroxylation and oxidation to form dopaquinone, the key intermediate in melanin biosynthesis.

**Why the Correct Answer is Right**
Tyrosine is the direct precursor of melanin. It is first converted to L-dopa by the enzyme tyrosine hydroxylase, then to dopaquinone via dopa decarboxylase. Dopaquinone undergoes further rearrangements to form eumelanin or pheomelanin. This pathway is central to skin, hair, and eye pigmentation. No other amino acid is directly involved in this sequence.

**Why Each Wrong Option is Incorrect**
Option A: Phenylalanine is metabolized to tyrosine via phenylalanine hydroxylase, but it is not a direct precursor of melanin.
Option C: Tryptophan is a precursor of serotonin and niacin, and is involved in the kynurenine pathway, not melanin synthesis.
Option D: Histidine is metabolized to histamine and is involved in immune regulation, not pigment formation.

**Clinical Pearl / High-Yield Fact**
Tyrosine deficiency or impaired tyrosine hydroxylase activity can lead to albinism or hypopigmentation disorders. Remember: **"Tyrosine → Melanin"** is a fixed pathway in biochemistry.

✓ Correct Answer: B. Tyrosine