**Core Concept**
Glycine is a non-essential, non-polar amino acid that plays a crucial role in various physiological processes. It serves as a neurotransmitter in the central nervous system, inhibits N-methyl-D-aspartate (NMDA) receptors, and is involved in the synthesis of purines, heme, and glutathione.

**Why the Correct Answer is Right**
Glycine inhibits NMDA receptors by binding to the glycine site, which is essential for receptor activation. This inhibition is crucial for preventing excessive glutamate release and excitotoxicity in the central nervous system. Glycine also acts as a co-agonist at NMDA receptors, facilitating the binding of glutamate and promoting synaptic plasticity. The inhibitory effects of glycine on NMDA receptors contribute to its role in regulating neuronal excitability and synaptic transmission.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycine is not a precursor to serotonin synthesis. Serotonin synthesis involves the conversion of tryptophan to 5-hydroxytryptophan, which is then converted to serotonin.

**Option B:** Glycine does not directly activate GABA receptors. While glycine and GABA are both inhibitory neurotransmitters, they act on distinct receptor subtypes. Glycine activates glycine receptors, whereas GABA activates GABA receptors.

**Option C:** Glycine is not primarily involved in the regulation of blood pressure. While glycine has various physiological effects, its primary role is not in blood pressure regulation.

**Clinical Pearl / High-Yield Fact**
Glycine's inhibitory effects on NMDA receptors make it a potential therapeutic target for treating conditions such as excitotoxicity, neurodegenerative diseases, and certain types of pain.

**Correct Answer:** D