Concept: This question tests your knowledge of the structural components of neurons, specifically the composition of Nissl bodies.

Why (D) is correct: Nissl bodies (also known as Nissl granules) are granular bodies found in the cytoplasm of neurons. They are primarily composed of free ribosomes and rough endoplasmic reticulum (RER). These structures are involved in protein synthesis, which is crucial for the neuron's growth, repair, and neurotransmitter production.

Why other options are wrong:

• (A) Nucleic acids and SER: While nucleic acids (RNA) are present in ribosomes, SER (smooth endoplasmic reticulum) is not a primary component of Nissl bodies. SER is involved in lipid synthesis and detoxification.
• (B) DNA and RNA: DNA is primarily found in the nucleus, not in Nissl bodies. While RNA (rRNA and mRNA) is a component of ribosomes, it's not the complete composition.
• (C) Proteins and lipids: While proteins are synthesized by Nissl bodies, and some lipids are present in membranes, this option is too general and doesn't specify the key structural components.

NEET Tip: Remember that the presence of abundant RER and ribosomes in Nissl bodies indicates a high rate of protein synthesis, which is essential for the metabolic activity of neurons.

Correct Answer: (D)