Which is correct ?

Options

A. Water reabsorption in descending limb and sodium reabsorption in ascending limb of Henle loop occur under similar conditions
B. Sodium reabsorption in ascending limb of loop and collecting duct occur under similar conditions
C. Water reabsorption in descending limb of loop and collecting duct occur under similar conditions
D. Water reabsorption in descending limb of loop and collecting duct occur under different conditions

Answer

To determine which statement is correct regarding water resorption in the descending limb and sodium resorption in the ascending limb of the loop of Henle, we can follow these steps:

### Step 1: Understand the Structure of the Nephron
The nephron is the functional unit of the kidney, consisting of the Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle (which includes the descending and ascending limbs), distal convoluted tubule (DCT), and collecting duct.

### Step 2: Analyze the Loop of Henle
- **Descending Limb**: This part of the loop is permeable to water but not to sodium and other electrolytes. As the filtrate moves down this limb, water is reabsorbed, leading to a concentration of solutes (making the urine hypertonic).
- **Ascending Limb**: This part is impermeable to water but actively reabsorbs sodium and chloride ions. As the filtrate ascends, it becomes hypotonic due to the loss of these solutes.

### Step 3: Compare Conditions for Resorption
- In the **descending limb**, water resorption occurs under conditions where the surrounding interstitial fluid is hypertonic, allowing water to move out of the filtrate.
- In the **ascending limb**, sodium resorption occurs under conditions where the nephron actively transports sodium ions out of the filtrate, making the filtrate hypotonic.

### Step 4: Conclusion
The conditions for water resorption in the descending limb and sodium resorption in the ascending limb are different. Therefore, the statement that they occur under similar conditions is incorrect.

### Final Answer
The correct answer is **Option D**: Water resorption in the descending limb and sodium resorption in the ascending limb occur under different conditions.

---