Assertion : While on going down the loop of Henle, the filtrate becomes hypotonic.
Reason : The descending limb of loop of Henle is impermeable to both water and electrolytes.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "while going down the loop of Henle, the filtrate becomes hypotonic."
- Hypotonic means that the concentration of solutes in the filtrate is lower than that in the surrounding fluid.

2. **Understanding the Reason**:
- The reason states that "the descending limb of the loop of Henle is impermeable to both water and electrolytes."
- If the descending limb is impermeable to water, then water cannot leave the filtrate, which would affect the concentration of the filtrate.

3. **Analyzing the Loop of Henle**:
- The descending limb of the loop of Henle is actually permeable to water. This means that as the filtrate moves down, water is reabsorbed into the surrounding medullary interstitium, which is hypertonic.
- As water leaves the filtrate, the concentration of solutes in the filtrate increases, making it hypertonic, not hypotonic.

4. **Conclusion**:
- Since the assertion is incorrect (the filtrate does not become hypotonic; it becomes hypertonic) and the reason is also incorrect (the descending limb is not impermeable to water), both the assertion and the reason are false.

5. **Final Answer**:
- Thus, the correct option is that both the assertion and the reason are false.