**Core Concept**
Serotonin synthesis is a crucial biochemical process that involves the conversion of an amino acid precursor into a neurotransmitter. This process occurs primarily in the central nervous system, particularly in the raphe nuclei of the brainstem, and plays a vital role in regulating various physiological and psychological functions.

**Why the Correct Answer is Right**
Serotonin is synthesized from the amino acid tryptophan through a series of enzyme-catalyzed reactions. The first step involves the hydroxylation of tryptophan to form 5-hydroxytryptophan (5-HTP) by the enzyme tryptophan hydroxylase. This is followed by the decarboxylation of 5-HTP to form serotonin (5-hydroxytryptamine) by the enzyme aromatic L-amino acid decarboxylase. This process is essential for the production of serotonin, which then binds to various serotonin receptors to exert its effects on the body.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct amino acid precursor for serotonin synthesis.
**Option B:** This option is incorrect because it is not the correct amino acid involved in serotonin synthesis.
**Option C:** This option is incorrect because it does not accurately describe the amino acid precursor for serotonin synthesis.

**Clinical Pearl / High-Yield Fact**
The synthesis of serotonin from tryptophan is tightly regulated by various factors, including the availability of tryptophan, the activity of tryptophan hydroxylase, and the presence of inhibitors or activators of the serotonin synthesis pathway. This regulation is essential for maintaining normal serotonin levels and preventing excessive or deficient serotonin activity, which can lead to various neurological and psychiatric disorders.

**Correct Answer: D. Tryptophan**