The high osmotic concentration of the kidney medulla is maintained by all the following except:

Options

A. diffusion of salt from the ascending limb of the loop of Henle
B. active transport of salt from the upper region of the ascending limb
C. spatial arrangement of juxtamedullary nephrons
D. diffusion of salt from the collecting duct

Explanation

To solve the question regarding the maintenance of high osmotic concentration in the kidney medulla, we need to analyze each option provided and determine which one does not contribute to this process.

### Step-by-Step Solution:

1. **Understanding Kidney Structure**:
- The kidney consists of two main regions: the cortex (outer region) and the medulla (inner region). The nephron, the functional unit of the kidney, has a renal corpuscle and renal tubules.

2. **Loop of Henle**:
- The Loop of Henle is a U-shaped structure in the nephron that consists of a descending limb and an ascending limb. The descending limb is permeable to water, while the ascending limb is impermeable to water but permeable to salts.

3. **Analyzing the Options**:
- **Option 1**: Diffusion of salt from the ascending limb of the Loop of Henle.
- This option is correct because salts diffuse out of the ascending limb, contributing to the osmotic concentration in the medulla.

- **Option 2**: Active transport of salt from the upper region of the ascending limb.
- This option is also correct as active transport mechanisms help reabsorb salts, further increasing osmotic concentration.

- **Option 3**: Special arrangement of the juxtamedullary nephron.
- This option is correct because juxtamedullary nephrons have long loops that extend deep into the medulla, aiding in maintaining osmotic concentration.

- **Option 4**: Diffusion of salt from the collecting duct.
- This option is incorrect. The collecting duct primarily reabsorbs water and urea, and while it may have some salt reabsorption, it does not significantly contribute to the high osmotic concentration of the medulla.

4. **Conclusion**:
- The correct answer to the question is **the diffusion of salt from the collecting duct**, as it does not play a significant role in maintaining the high osmotic concentration of the kidney medulla.

### Final Answer:
The high osmotic concentration of the kidney medulla is maintained by all the following except: **the diffusion of salt from the collecting duct**.