Select the incorrect statement.

Options

A. The countercurrent mechanism changes the isotonic glomerular filtrate into hypertonic urine by increasing salt concentration around the nephron and collecting tubule.
B. The wall of collecting tubule is permeable to water whereas, ascending limb is impermeable to water.
C. The absorption of water in DCT is facultative.
D. As the filtrate passes through the ascending limb, sodium is transported passively in the ascending thick segment.

Answer

To solve the question of selecting the incorrect statement, we will analyze each of the four statements provided.

### Step-by-Step Solution:

1. **Statement Analysis**:
- **Statement A**: "The counter current mechanism changes the isotonic glomerular filtrate into hypertonic urine by increasing salt concentration around the nephron and collecting tubule."
- This statement is **correct**. The counter current mechanism indeed helps in concentrating urine by creating a gradient of salt concentration.

2. **Statement B**: "Wall of collecting tubule is permeable to water whereas ascending limb is impermeable to water."
- This statement is also **correct**. The collecting tubule is permeable to water, allowing for water reabsorption, while the ascending limb of the loop of Henle is impermeable to water.

3. **Statement C**: "Absorption of water in DCT is a facultative mechanism."
- This statement is **correct**. The distal convoluted tubule (DCT) does allow for the facultative reabsorption of water, depending on the body's needs and the presence of hormones like ADH.

4. **Statement D**: "As the filtrate passes through the ascending limb, sodium is transported passively in the ascending thick segment."
- This statement is **incorrect**. Sodium is transported actively in the ascending limb of the loop of Henle, specifically in the thick ascending limb, where active transport mechanisms are employed to move sodium ions out of the filtrate.

### Conclusion:
The incorrect statement among the options is **Statement D**.