The nephron is a hollow, convoluted tube of cells. It is engineered to concentrate urine by removing water at which of the following sites?
I. Proximal convoluted tubule
II. Descending limb of the loop of Henle
III. Ascending limb of the loop of Henle

Options

A. I only
B. II only
C. I and II only
D. I, II and III

Answer

To determine at which sites the nephron concentrates urine by removing water, we need to analyze the function of each part of the nephron mentioned in the question.

### Step-by-Step Solution:

1. **Understanding the Nephron Structure**:
- The nephron consists of several parts: Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle (which includes the descending limb and ascending limb), distal convoluted tubule (DCT), and collecting ducts.

2. **Proximal Convoluted Tubule (PCT)**:
- The PCT is responsible for the reabsorption of a significant amount of water, along with nutrients and ions. Approximately 65% of the filtered water is reabsorbed here. This process is crucial for concentrating urine.

3. **Descending Limb of the Loop of Henle**:
- The descending limb is permeable to water but not to solutes. As the filtrate moves down this limb, water is reabsorbed into the surrounding medullary interstitium, leading to an increase in the concentration of the filtrate. This also contributes to urine concentration.

4. **Ascending Limb of the Loop of Henle**:
- The ascending limb is impermeable to water but actively transports sodium and chloride ions out of the filtrate. This segment does not contribute to water reabsorption, but it plays a role in creating a concentration gradient in the medulla, which is essential for the concentration of urine in the collecting ducts.

5. **Conclusion**:
- The nephron concentrates urine primarily at the Proximal Convoluted Tubule and the Descending Limb of the Loop of Henle. The Ascending Limb does not remove water, so it does not contribute to urine concentration.

### Final Answer:
The nephron is engineered to concentrate urine by removing water at:
- I. Proximal convoluted tubule
- II. Descending limb of the loop of Henle