**Core Concept**
Serotonin synthesis in the human body involves the hydroxylation of tryptophan to 5-hydroxytryptophan (5-HTP), which is then converted to serotonin through decarboxylation. This process is mediated by the enzyme tryptophan hydroxylase.

**Why the Correct Answer is Right**
Tryptophan is the essential amino acid precursor to serotonin synthesis. The enzyme tryptophan hydroxylase specifically catalyzes the hydroxylation of tryptophan at the 5-position, producing 5-HTP. This step is crucial for the subsequent conversion to serotonin. The correct answer is supported by the fact that tryptophan is the only amino acid among the options that can be converted to serotonin.

**Why Each Wrong Option is Incorrect**
**Option B:** Glycine is an amino acid involved in the synthesis of heme, creatine, and glutathione, but it is not a precursor to serotonin synthesis.

**Option C:** Threonine is involved in the synthesis of ascorbic acid and is a precursor to glycine and serine, but it is not directly involved in serotonin synthesis.

**Option D:** Methionine is involved in the synthesis of cysteine, taurine, and glutathione, but it is not a precursor to serotonin synthesis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that tryptophan is the only essential amino acid precursor to serotonin synthesis, and its availability can influence mood regulation and other physiological processes.

**✓ Correct Answer: A. Tryptophan**