Which of the following is not a function of the mammalian kidney ?

Options

A. Water retention
B. Regulation of sodium in the blood
C. Excertion of toxins
D. Synthesis of urea

Answer

### Step-by-Step Solution:

1. **Understanding Kidney Functions**: The mammalian kidney performs several essential functions, including water reabsorption, regulation of sodium levels, and excretion of waste products.

2. **Analyzing Each Option**:
- **Water Retention**: The kidneys are responsible for reabsorbing water. This process occurs in various parts of the nephron, including the Proximal Convoluted Tubule (PCT), Distal Convoluted Tubule (DCT), Loop of Henle, and Collecting Duct. Therefore, this is a function of the kidney.
- **Regulation of Sodium**: The kidneys also regulate sodium levels in the blood. Sodium reabsorption primarily occurs in the PCT. This is another function of the kidney.
- **Excretion of Toxins**: The kidneys filter out toxins and waste products such as urea and creatinine from the blood, which are then excreted in urine. This is a key function of the kidneys.
- **Synthesis of Urea**: This is where the confusion may arise. The mammalian kidney does not synthesize urea; instead, urea is produced in the liver through the urea cycle. The kidneys are responsible for excreting urea, not synthesizing it.

3. **Conclusion**: After analyzing all the options, the function that is **not** performed by the mammalian kidney is the **synthesis of urea**. Therefore, the correct answer is the last option.

### Final Answer:
The function that is not performed by the mammalian kidney is the **synthesis of urea**.

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