Which of the following structure is not made up of a single layer of flattened cell with irregular boundaries?

Options

A. Alveoli
B. Endothelium
C. Bowman's capsule
D. Skin

Answer

To solve the question, "Which of the following structure is not made up of a single layer of flattened cell with irregular boundaries?", we will analyze each of the given options based on their cellular structure.

### Step-by-Step Solution:

1. **Understand the Question**: We need to identify a structure that does not consist of a single layer of flattened cells with irregular boundaries. Such structures typically allow for easy diffusion.

2. **Examine the Options**:
- **Option 1: Alveoli**:
- Alveoli are tiny air sacs in the lungs where gas exchange occurs. They are made up of a single layer of flattened epithelial cells (simple squamous epithelium), which facilitates the diffusion of oxygen and carbon dioxide. Therefore, this option does not fit our criteria.

- **Option 2: Endothelium**:
- The endothelium is a thin layer of cells lining the blood vessels and heart. It is also composed of a single layer of flattened cells, allowing for efficient exchange of substances. Thus, this option is also not our answer.

- **Option 3: Bowman's Capsule**:
- Bowman's capsule is part of the nephron in the kidney and is involved in the filtration of blood. It consists of a single layer of flattened cells, which is necessary for filtration. Hence, this option does not meet the criteria either.

- **Option 4: Skin**:
- The skin is a protective barrier that consists of multiple layers: the epidermis, dermis, and subcutaneous layer. It is not made up of a single layer of cells; rather, it has several layers of cells, making it the correct answer to our question.

3. **Conclusion**: Based on the analysis, the structure that is not made up of a single layer of flattened cells with irregular boundaries is **Option 4: Skin**.

### Final Answer:
**Option 4: Skin**