Assertion `:-` Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed.
Reason `:-` Counter current mechanism helps to maintain a concentration gradient in the medullary interstitium.

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

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 human kidneys can produce urine that is nearly four times more concentrated than the initial filtrate formed.
- The initial filtrate is formed in the Bowman's capsule and is relatively dilute. As it passes through the nephron, particularly the collecting duct, water is reabsorbed, leading to a more concentrated urine.

2. **Analyzing the Reason**:
- The reason given is that the countercurrent mechanism helps maintain a concentration gradient in the medullary interstitium.
- The countercurrent mechanism refers to the flow of filtrate in opposite directions in the nephron (specifically in the loop of Henle) and the blood flow in the vasa recta. This mechanism is crucial for creating a concentration gradient that allows for the reabsorption of water.

3. **Connecting Assertion and Reason**:
- The assertion is true because the kidneys can concentrate urine significantly through the reabsorption of water in the collecting ducts.
- The reason is also true because the countercurrent mechanism is essential for establishing the concentration gradient that facilitates this water reabsorption.

4. **Conclusion**:
- Since both the assertion and reason are true, and the reason correctly explains the assertion, we conclude that the correct answer is that both statements are true and the reason is the correct explanation of the assertion.

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