**Step-by-Step Solution:**
1. **Understanding Brush Border**: The term "brush border" refers to the microvilli present on the plasma membrane of certain epithelial cells. These microvilli increase the surface area of the cell, enhancing its ability to absorb substances.
2. **Identifying the Options**: The question provides four options regarding where the brush border is characteristic:
- Neck of nephron
- Collecting tube
- Proximal convoluted tubule
- All of the above
3. **Analyzing Each Option**:
- **Neck of Nephron**: This segment connects the Bowman's capsule to the proximal tubule. It does not have a brush border; instead, it contains thin filaments.
- **Collecting Tube**: This is the final section of the nephron that leads to the ureter. It is not specialized for absorption and does not have a brush border.
- **Proximal Convoluted Tubule (PCT)**: This part of the nephron is lined with simple cuboidal epithelium that has a prominent brush border. This structure significantly increases the surface area for reabsorption of nutrients and electrolytes.
4. **Conclusion**: Based on the analysis, the brush border is characteristic of the proximal convoluted tubule (PCT), as it plays a crucial role in the absorption of essential nutrients and electrolytes.
5. **Final Answer**: The brush border is characteristic of the proximal convoluted tubule.
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