A nephron's reabsorption mechanism depends on:

Options

A. osmosis across nephron wall
B. active transport of sodium across nephron wall
C. a steep solute concentration gradient
D. all of the above

Answer

To answer the question regarding what a nephron's reabsorption mechanism depends on, we can break down the components involved in the reabsorption process step by step.

### Step-by-Step Solution:

1. **Understanding Nephron Structure**:
- A nephron consists of several parts including the glomerulus, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct. Each part plays a role in the filtration and reabsorption of substances.

2. **Reabsorption Mechanism**:
- Reabsorption in the nephron is the process by which essential substances (like water, ions, and nutrients) are reabsorbed back into the bloodstream after being filtered out from the blood.

3. **Osmosis Across Nephron Wall**:
- Osmosis is the movement of water across a semipermeable membrane. In the nephron, water is reabsorbed from the filtrate back into the blood, which is crucial for maintaining fluid balance and concentration of solutes.

4. **Active Transport of Sodium**:
- Active transport mechanisms are used to move sodium ions (Na+) from the nephron tubules back into the blood. This process requires energy (ATP) and is vital for maintaining electrolyte balance and creating a concentration gradient.

5. **Steep Solute Concentration Gradient**:
- A steep concentration gradient of solutes (like sodium and chloride ions) is necessary for effective reabsorption. This gradient facilitates the movement of water and other solutes from the nephron back into the blood.

6. **Conclusion**:
- The nephron's reabsorption mechanism depends on all of the above factors: osmosis, active transport of sodium, and the steep solute concentration gradient. Therefore, the correct answer is option D: all of the above.

### Final Answer:
The nephron's reabsorption mechanism depends on:
- A) Osmosis across nephron wall
- B) Active transport of sodium across the nephron wall
- C) Steep solute concentration gradient
- D) All of the above (Correct Answer)