## **Core Concept**
Nissl bodies, also known as Nissl substance or tigroid bodies, are characteristic structures found in the cytoplasm of neurons. They are composed of rough endoplasmic reticulum (RER) and ribosomes, which are essential for protein synthesis. The presence and appearance of Nissl bodies can be crucial for identifying and classifying neurons.

## **Why the Correct Answer is Right**
The correct answer, **D. Rough endoplasmic reticulum and ribosomes**, is right because Nissl bodies are indeed comprised of rough endoplasmic reticulum (RER) and free ribosomes. These organelles are vital for the synthesis of proteins within the neuron. The RER, studded with ribosomes, is responsible for producing proteins that are often destined for secretion or for insertion into cellular membranes. The abundance and distribution of Nissl bodies can vary depending on the type of neuron and its functional state.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately describe Nissl bodies.
- **Option B:** This option is incorrect as it does not correctly identify the composition of Nissl bodies.
- **Option C:** This option is incorrect because, similar to options A and B, it does not accurately represent what Nissl bodies are.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Nissl bodies are typically absent in the axon hillock and initial segment of the neuron, which is a region critical for the generation of action potentials. This absence is significant because it correlates with the functional and structural specialization of different parts of the neuron.

## **Correct Answer:** D. Rough endoplasmic reticulum and ribosomes