**Core Concept**
Excitatory neurotransmitters are chemical messengers in the nervous system that increase the likelihood of a neuron firing or transmitting a signal to another neuron. They play a crucial role in various physiological and pathological processes, including learning and memory, mood regulation, and muscle contraction.

**Why the Correct Answer is Right**
Excitatory neurotransmitters work by binding to specific receptors on the postsynaptic neuron, which triggers an increase in the concentration of cAMP or Ca2+ ions, leading to the opening of voltage-gated channels and ultimately, neuronal excitation. Examples of excitatory neurotransmitters include glutamate, aspartate, acetylcholine, and dopamine.

**Why Each Wrong Option is Incorrect**
* **Option A:** GABA (Gamma-Aminobutyric Acid) is actually an inhibitory neurotransmitter, which means it decreases the likelihood of a neuron firing. It works by binding to GABA receptors, which opens chloride channels and hyperpolarizes the neuron.
* **Option B:** This option is missing, so we'll skip to the next one.
* **Option C:** This option is missing, so we'll skip to the next one.
* **Option D:** This option is missing, so we'll skip to the next one.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the balance between excitatory and inhibitory neurotransmitters is crucial for maintaining normal neuronal function. An imbalance between these two types of neurotransmitters has been implicated in various neurological and psychiatric disorders, including epilepsy, anxiety, and depression.

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