## **Core Concept**
The question tests understanding of cellular structures, specifically their composition and membrane association. Eukaryotic cells have various membrane-bound organelles, but not all intracellular structures are enveloped by a cell membrane.

## **Why the Correct Answer is Right**
The correct answer, , refers to **Ribosomes**. Ribosomes are small organelles found throughout the cytoplasm, on the endoplasmic reticulum, or attached to the nuclear membrane, and are responsible for protein synthesis. Unlike many other organelles such as the endoplasmic reticulum, Golgi apparatus, lysosomes, and mitochondria, ribosomes themselves do not have a membrane.

## **Why Each Wrong Option is Incorrect**
- **Option A:** . This likely refers to the **Endoplasmic Reticulum (ER)**, which is a type of organelle that forms an interconnected network of flattened, membrane-enclosed sacs or tube-like structures. The ER is indeed enveloped by a cell membrane.
- **Option B:** . This could refer to **Mitochondria**, often termed the "powerhouses" of the cell. Mitochondria generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. Mitochondria have two membranes: an outer and an inner membrane.
- **Option D:** . This might refer to **Lysosomes**, which are membrane-bound organelles responsible for cellular digestion of macromolecules, using enzymes that are acidic in nature. Lysosomes are enclosed by a single membrane.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while many organelles are membrane-bound, ribosomes are not. This distinction is crucial for understanding cellular function and pathology. For example, in cells infected by certain viruses or in certain diseases like **I-cell disease**, the abnormal targeting and functioning of lysosomes (which are membrane-bound) can lead to cellular dysfunction.

## **Correct Answer: C. Ribosomes**