A mammal excretes nitrogen in the form of:

Options

A. ammonium ions
B. amino acids
C. uric acid
D. urea

Explanation

**Step-by-Step Solution:**

1. **Understand the Question**: The question asks about the form in which mammals excrete nitrogen. This means we need to identify the nitrogenous waste product that mammals release.

2. **Review the Options**: The options provided are:
- A. Ammonium ions
- B. Amino acids
- C. Uric acids
- D. Urea

3. **Analyze Each Option**:
- **Ammonium ions**: These are typically not excreted by mammals. They are more commonly found in aquatic organisms that can afford to lose ammonia directly into the water.
- **Amino acids**: While amino acids are components of proteins, they are not excreted directly as waste. Instead, they are usually deaminated to remove the nitrogen before further processing.
- **Uric acids**: This form of nitrogenous waste is primarily excreted by birds and reptiles, not mammals.
- **Urea**: This is the primary nitrogenous waste product excreted by mammals. It is produced in the liver through the urea cycle and then transported to the kidneys for excretion.

4. **Identify the Correct Answer**: Since mammals primarily excrete nitrogen in the form of urea, the correct answer is D. Urea.

5. **Conclusion**: Therefore, the answer to the question is that a mammal excretes nitrogen in the form of urea.

**Final Answer**: D. Urea

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