Which feature enables the mammalian kidney to concentrate urine in the medullary region?
(a) Rapid removal of sodium ions from medullary tissues.
(b) Maintaining a high osmotic pressure in the tissues between the tubules.
(c) High oxidative metabolism of medullary cells.
(d) Rapid flow of blood through the medulla.

Options

A. Rapid removal of sodium ions from medullary tissues.
B. Maintaining a high osmotic pressure in the tissues between the tubules.
C. High oxidative metabolism of medullary cells.
D. Rapid flow of blood through the medulla.

Answer

To solve the question regarding which feature enables the mammalian kidney to concentrate urine in the medullary region, we can analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding the Function of the Kidney:**
The primary function of the kidney is to filter blood, remove waste, and concentrate urine. The medullary region of the kidney plays a crucial role in concentrating urine.

2. **Analyzing Option (a): Rapid removal of sodium ions from medullary tissues.**
- While sodium ions are important for creating osmotic gradients, the rapid removal alone does not explain how urine concentration occurs. This option does not directly relate to maintaining osmotic pressure.

3. **Analyzing Option (b): Maintaining a high osmotic pressure in the tissues between the tubules.**
- This option highlights the importance of osmotic pressure. The kidney's ability to concentrate urine is largely due to the high osmotic pressure maintained in the medullary interstitial fluid. This osmotic gradient allows for water reabsorption from the collecting ducts, leading to concentrated urine.

4. **Analyzing Option (c): High oxidative metabolism of medullary cells.**
- While oxidative metabolism is essential for energy production in kidney cells, it does not directly relate to the concentration of urine. This option does not address the osmotic gradient necessary for urine concentration.

5. **Analyzing Option (d): Rapid flow of blood through the medulla.**
- A rapid blood flow may not support the concentration of urine effectively, as it could wash away the necessary solutes that create the osmotic gradient. This option is less relevant to the concentration process.

6. **Conclusion:**
After analyzing all options, it is clear that the correct answer is **(b) Maintaining a high osmotic pressure in the tissues between the tubules.** This high osmotic pressure is crucial for the kidney's ability to concentrate urine in the medullary region.

### Final Answer:
(b) Maintaining a high osmotic pressure in the tissues between the tubules.