Hypertonic urine formation is a characteristic of kidneys of

Options

A. Fishes and amphibians
B. Amphibians and reptiles
C. Reptiles and fishes
D. Birds and mammals

Answer

To solve the question "Hypertonic urine formation is a characteristic of kidneys of", we will analyze the options provided and the characteristics of urine formation in different groups of animals.

### Step-by-Step Solution:

1. **Understanding Hypertonic Urine**:
- Hypertonic urine is concentrated urine where water is reabsorbed, leading to a higher concentration of solutes. This occurs when the kidneys conserve water, resulting in urine that has a higher osmolarity than blood plasma.

2. **Analyzing the Options**:
- **Option 1: Fishes and Amphibians**:
- Most fishes excrete ammonia directly into the water, and many amphibians, when in water, also excrete ammonia. Amphibians can produce urea when on land, but they do not primarily produce hypertonic urine.
- **Option 2: Amphibians and Reptiles**:
- Amphibians primarily produce hypotonic urine, especially when in an aquatic environment. Reptiles, on the other hand, do produce uric acid, which is less water-soluble, but they do not primarily produce hypertonic urine.
- **Option 3: Reptiles and Fishes**:
- As mentioned, fishes do not produce hypertonic urine, and reptiles primarily excrete uric acid.
- **Option 4: Birds and Mammals**:
- Birds and mammals have highly developed kidneys (nephrons with loops of Henle) that allow for the production of hypertonic urine. The countercurrent mechanism in the loop of Henle and the vasa recta enables them to concentrate urine effectively.

3. **Conclusion**:
- Based on the analysis, the correct answer is **Option 4: Birds and Mammals**. They are capable of forming hypertonic urine due to their advanced kidney structures and mechanisms.

### Final Answer:
Hypertonic urine formation is a characteristic of kidneys of **birds and mammals**.

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