Reabsorption of useful substances back into blood from the filtrate in a nephron occurs in:

Options

A. proximal convoluted tubule
B. distal convoluted tubule
C. collecting duct
D. loop of Henle

Explanation

To solve the question regarding where the reabsorption of useful substances back into the blood from the filtrate occurs in a nephron, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Nephron Structure**:
- The nephron is the functional unit of the kidney and consists of several parts: Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.

2. **Identifying the Function of Each Part**:
- Each part of the nephron has specific functions:
- **Bowman's Capsule**: Filters blood to form filtrate.
- **Proximal Convoluted Tubule (PCT)**: Primarily responsible for the reabsorption of useful substances.
- **Loop of Henle**: Concentrates urine and reabsorbs water and salts.
- **Distal Convoluted Tubule (DCT)**: Further adjusts the composition of urine.
- **Collecting Duct**: Final adjustments to urine concentration and volume.

3. **Focusing on the PCT**:
- The PCT is lined with cuboidal brush border epithelium, which increases the surface area for reabsorption.
- It reabsorbs a significant amount of useful substances such as glucose, amino acids, and ions (like sodium) from the filtrate back into the blood.

4. **Conclusion**:
- Based on the functions of the nephron parts, the reabsorption of useful substances back into the blood primarily occurs in the **Proximal Convoluted Tubule (PCT)**.

### Final Answer:
The reabsorption of useful substances back into the blood from the filtrate in a nephron occurs in the **Proximal Convoluted Tubule (PCT)**.

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