## Core Concept
The question tests the understanding of amino acid metabolism, specifically the synthesis pathways of nicotinic acid (niacin or vitamin B3) and serotonin (5-hydroxytryptamine). The core concept revolves around identifying the common precursor for these two compounds.

## Why the Correct Answer is Right
The correct answer, **Tryptophan**, is an essential amino acid that serves as a precursor for several important molecules. Tryptophan is converted into **kynurenine** through the kynurenine pathway, which can then be further metabolized into **nicotinic acid** (or niacin). Additionally, tryptophan is hydroxylated to form **5-hydroxytryptophan**, which is then decarboxylated to form **serotonin (5-HT)**. This dual role of tryptophan makes it crucial for both niacin synthesis and neurotransmitter production.

## Why Each Wrong Option is Incorrect
- **Option A: Tyrosine** is incorrect because, although it is another amino acid, it is primarily involved in the synthesis of catecholamines (dopamine, norepinephrine, and epinephrine) and not directly related to the synthesis of nicotinic acid or serotonin.
- **Option B: Glutamate** is incorrect as it is a key player in nitrogen metabolism and can be converted into several amino acids and neurotransmitters but is not directly involved in the synthesis of nicotinic acid or serotonin.
- **Option D: Glycine** is incorrect because, while it is a precursor for purine synthesis, heme synthesis, and can be converted into glutathione, it does not directly contribute to the synthesis of nicotinic acid or serotonin.

## Clinical Pearl / High-Yield Fact
A memorable point for exams is that tryptophan deficiency can lead to pellagra (a condition caused by niacin deficiency) and that alterations in tryptophan metabolism can affect serotonin levels, which are implicated in various psychiatric conditions. A classic clinical correlation is the carcinoid syndrome, where there's an overproduction of serotonin.

## Correct Answer: C. Tryptophan