## Core Concept
Neurotransmitters are chemical messengers that transmit signals across synapses, enabling communication between neurons. They play a crucial role in various physiological processes, including mood regulation, muscle contraction, and cognition. The characteristics of neurotransmitters include synthesis, storage, release, and reuptake or degradation.

## Why the Correct Answer is Right
The correct answer, ., implies that one of the options does not fit with the typical features of neurotransmitters. Features of neurotransmitters include:
- Being synthesized and stored in neurons
- Being released into the synaptic cleft upon neuronal stimulation
- Binding to specific receptors on the postsynaptic neuron
- Undergoing reuptake or degradation to terminate the signal

## Why Each Wrong Option is Incorrect
**Option A:** - This option is not provided, but generally, features such as synthesis, release, and binding to receptors are characteristics of neurotransmitters.

**Option B:** - Similarly, without the specific details of Option B, we consider typical features like modulation of postsynaptic neuron activity.

**Option C:** - This option is also not detailed, but if it aligns with known neurotransmitter functions, it would be incorrect as an "except" choice.

**Option D:** - Given that . is the correct answer, Option D likely describes a feature not typical of neurotransmitters or is less directly related to their primary function.

## Clinical Pearl / High-Yield Fact
A key point to remember is that neurotransmitters can have various effects on postsynaptic neurons, including excitatory, inhibitory, or modulatory actions. This diversity in action is crucial for understanding neurological and psychiatric disorders, as well as the mechanisms of drugs used to treat these conditions.

## Correct Answer: D.