Assertion `:-` Peritubular capillaries secrete substances in the convoluted tubules.
Reason `:-` Harmful and useless substances which escaped ultrafiltration are removed by active secretion.

Options

A. If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion .
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertion
C. If Assertion is True but the Reason is False.
D. If both Assertion & Reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Peritubular capillaries secrete substances in the convoluted tubules."
- **Explanation**: Peritubular capillaries are small blood vessels that surround the nephron tubules in the kidney. They play a crucial role in the reabsorption and secretion processes that occur in the nephron.

2. **Understanding the Reason**: The reason states that "Harmful and useless substances which escaped ultrafiltration are removed by active secretion."
- **Explanation**: During the process of ultrafiltration in the kidneys, certain substances may not be filtered out and remain in the bloodstream. The kidneys utilize active secretion to remove these substances from the blood into the tubular fluid, thus eliminating harmful or unnecessary components.

3. **Linking Assertion and Reason**: Both the assertion and the reason are true, and the reason correctly explains why the assertion is true.
- **Explanation**: The peritubular capillaries do indeed secrete substances into the convoluted tubules, and this process is essential for the removal of harmful substances that were not filtered out during ultrafiltration.

4. **Conclusion**: Since both the assertion and the reason are true, and the reason provides a valid explanation for the assertion, we can conclude that the correct answer is option number 1.

### Final Answer:
Both the assertion and reason are true, and the reason is the correct explanation for the assertion.

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