Which of the following setences about vasa recta is correct ?

Options

A. It is a part of the renal tubule.
B. It is a U-shaped vessel running parallel to loop of Henle deep into the medulla in most of the nephrons.
C. It originates from the afferent arteriole.
D. During the counter-current mechanism, NaCl is transpoted back to the interstitium by the ascending portion of vasa recta.

Answer

To solve the question about which statement regarding vasa recta is correct, we will analyze each option provided.

### Step-by-Step Solution:

1. **Understand the Structure of Vasa Recta**:
- Vasa recta are specialized blood vessels in the kidneys that play a crucial role in the countercurrent exchange mechanism. They are straight arterioles and venules that run parallel to the loop of Henle.

2. **Evaluate the First Option**:
- The first option states that vasa recta is part of the renal tubule.
- **Analysis**: This statement is incorrect because vasa recta are not part of the renal tubule; they are blood vessels associated with the nephron.

3. **Evaluate the Second Option**:
- The second option claims that vasa recta are U-shaped vessels running parallel to the loop of Henle.
- **Analysis**: This statement is also incorrect. Vasa recta are straight vessels, not U-shaped, and they do run parallel to the loop of Henle.

4. **Evaluate the Third Option**:
- The third option states that vasa recta originate from the afferent arteriole.
- **Analysis**: This statement is incorrect as well. Vasa recta originate from the efferent arterioles of the juxtamedullary nephrons, not the afferent arterioles.

5. **Evaluate the Fourth Option**:
- The fourth option mentions that during the countercurrent mechanism, NaCl is transported back into the interstitium by the ascending portion of the vasa recta.
- **Analysis**: This statement is correct. The ascending portion of the vasa recta does play a role in the countercurrent exchange by reabsorbing NaCl back into the interstitium.

### Conclusion:
The correct answer is **Option 4**: "During the countercurrent mechanism, NaCl is transported back into the interstitium by the ascending portion of the vasa recta."

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