All mammals, without any exception, are characterised by

Options

A. Biconcave red blood cells
B. Extra-abdominal testes and four chambered heart
C. Heterdont teeth and twelve pairs of cranial nerves
D. A muscular diaphragm

Answer

To solve the question "All mammals, without any exception, are characterised by," we need to analyze each of the provided options to identify which characteristic is exclusive to mammals.

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for a characteristic that is unique to all mammals without any exceptions.

2. **Evaluate Option A - Biconcave Red Blood Cells**:
- Biconcave red blood cells are indeed a feature of most mammals.
- However, camels have oval-shaped red blood cells, which means this characteristic is not universal among all mammals.
- **Conclusion**: This option is incorrect.

3. **Evaluate Option B - Extra Abdominal Testes and Four Chambered Heart**:
- Extra abdominal testes are typically found in mammals to maintain a lower temperature for sperm production, but some mammals like elephants and whales have internal testes.
- A four-chambered heart is also found in birds and crocodiles, making it not exclusive to mammals.
- **Conclusion**: This option is incorrect.

4. **Evaluate Option C - Heterodont Teeth and 12 Pairs of Cranial Nerves**:
- Heterodont teeth (different types of teeth for different functions) are common in mammals, but prototherians (like the platypus) lack true teeth.
- While 12 pairs of cranial nerves are present in mammals, the presence of heterodont teeth is not universal due to exceptions.
- **Conclusion**: This option is incorrect.

5. **Evaluate Option D - Muscular Diaphragm**:
- The muscular diaphragm is a primary respiratory muscle that separates the thoracic and abdominal cavities.
- This structure is exclusively found in mammals and is not present in any other group of organisms.
- **Conclusion**: This option is correct.

### Final Answer:
The correct answer is **Option D - Muscular Diaphragm**.

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