**Question:** The following is an inhibitory agent in neuron:

A. GABA (Gamma-Aminobutyric Acid)
B. Serotonin
C. Choline
D. Acetylcholine

**Correct Answer:** A. GABA (Gamma-Aminobutyric Acid)

**Core Concept:**
Inhibition in neurons refers to the decrease in the rate or extent of a physiological response, usually due to the action of neurotransmitters. In the central nervous system (CNS), the primary inhibitory neurotransmitter is GABA (Gamma-Aminobutyric Acid). GABA acts on GABA receptors to decrease the likelihood of an action potential occurring in the postsynaptic neuron, thus inhibiting its activity.

**Why the Correct Answer is Right:**
GABA is the primary inhibitory neurotransmitter in the CNS. It achieves this inhibitory effect by binding to GABA receptors on the postsynaptic neuron. These receptors are classified into two main types: GABA-A receptors (ionotropic receptors) and GABA-B receptors (metabotropic receptors). GABA-A receptors are ligand-gated chloride channels, which leads to an influx of chloride ions into the cell, resulting in hyperpolarization and a decrease in the neuron's excitability. This hyperpolarization reduces the likelihood of an action potential occurring and thus inhibits the neuron's activity.

**Why Each Wrong Option is Incorrect:**
1. Serotonin (B): Serotonin is primarily involved in regulating mood, appetite, sleep, and other physiological processes. While it does have inhibitory effects in some areas like the gastrointestinal tract, its primary role is not in the CNS as an inhibitory neurotransmitter.
2. Choline (C): Choline is a precursor to the neurotransmitter acetylcholine and is essential for the synthesis of phosphatidylcholine, a major component of cell membranes. It does not play a significant role in neuronal signaling as an inhibitory neurotransmitter in the context of the question.
3. Acetylcholine (D): Acetylcholine is a neurotransmitter that plays a crucial role in the transmission of nerve impulses at neuromuscular junctions, not in the CNS as an inhibitory neurotransmitter. Inhibitory neurotransmitters are primarily GABA and glycine in the CNS.

**Clinical Pearl:**
Inhibitory neurotransmission in the CNS primarily involves two neurotransmitters: GABA (Gamma-aminobutyric acid) and glycine. These neurotransmitters are responsible for decreasing neuronal excitability and maintaining overall neuronal stability. Understanding the role of these neurotransmitters and their receptors in neuronal signaling is essential for understanding neurological functions, pathologies, and pharmacology.