Urine is always fluid except in

Options

A. Reptiles and amphibians
B. Birds and reptiles
C. Birds and mammals
D. Reptiles and mammals

Answer

To solve the question "Urine is always fluid except in," we need to analyze the options provided and understand the types of nitrogenous waste excretion in different animal groups.

### Step-by-Step Solution:

1. **Identify the Types of Nitrogenous Waste:**
- Animals excrete nitrogenous wastes in different forms: ammonia, urea, and uric acid.
- Ammonotelic animals excrete ammonia, ureotelic animals excrete urea, and uricotelic animals excrete uric acid.

2. **Understand the Characteristics of Uric Acid:**
- Uric acid is the least soluble in water compared to ammonia and urea.
- Animals that excrete uric acid tend to conserve water, which is crucial for survival in arid environments.

3. **Analyze the Options:**
- **Reptiles and Amphibians:** Reptiles are uricotelic (excrete uric acid), but amphibians are ammonotelic (excrete ammonia). Therefore, this option is incorrect.
- **Birds and Reptiles:** Both birds and reptiles are uricotelic and excrete uric acid, which forms a paste-like substance rather than fluid urine. This option is correct.
- **Birds and Mammals:** Birds are uricotelic, but mammals are ureotelic (excrete urea). Thus, this option is incorrect.
- **Reptiles and Mammals:** Reptiles are uricotelic, but mammals are ureotelic. Hence, this option is also incorrect.

4. **Conclusion:**
- The correct answer is **birds and reptiles** because both groups excrete uric acid, resulting in a non-fluid excretion.

### Final Answer:
Urine is always fluid except in **birds and reptiles**.

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