Assertion: In the descending limb of loop of Henle the urine is hypotonic, while in ascending limb of loop of Henle, the urine is hypertonic.
Reason: Descending limb is impermeable to water while ascending limb is impermeable to `Na^(+)`

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 assertion and reason question, let's break down the statements step by step:

### Step 1: Understand the Structure of the Nephron
- The nephron consists of several parts: Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle (which includes the descending and ascending limbs), distal convoluted tubule (DCT), and collecting duct.
- The loop of Henle plays a crucial role in the concentration and dilution of urine.

**Hint:** Familiarize yourself with the nephron's structure and the flow of filtrate through its various parts.

### Step 2: Analyze the Assertion
- The assertion states that in the descending limb of the loop of Henle, the urine is hypotonic, while in the ascending limb, it is hypertonic.
- The descending limb is primarily permeable to water, allowing water to be reabsorbed into the bloodstream. As water is reabsorbed, the concentration of solutes in the filtrate increases, making it hypertonic, not hypotonic.

**Hint:** Remember that the descending limb allows water to leave, concentrating the filtrate.

### Step 3: Analyze the Reason
- The reason states that the descending limb is impermeable to water, while the ascending limb is impermeable to sodium ions (Na+).
- This statement is incorrect. The descending limb is permeable to water, and the ascending limb is impermeable to water but actively reabsorbs sodium and chloride ions.

**Hint:** Check the permeability characteristics of each limb of the loop of Henle.

### Step 4: Conclusion
- Both the assertion and the reason are false:
- The assertion is false because the urine in the descending limb is hypertonic, not hypotonic.
- The reason is false because the descending limb is permeable to water, and the ascending limb is not impermeable to sodium; it actively reabsorbs it.

**Final Answer:** Both the assertion and reason statements are false.