Which of the following substances are removed from blood in the kidneys?

Options

A. Water
B. Urea
C. Sodium
D. Ammonia

Explanation

To determine which substances are removed from blood in the kidneys, we can analyze the options provided. Here’s a step-by-step solution:

### Step 1: Understand the function of kidneys
The kidneys are responsible for filtering blood, removing waste products, and regulating the balance of electrolytes and fluids in the body.

### Step 2: Analyze the options
1. **Water**: While the kidneys do filter water, they primarily reabsorb it to maintain fluid balance. Therefore, water is not removed but rather reabsorbed.

2. **Urea**: Urea is a waste product formed from the breakdown of proteins. It is indeed removed from the blood in the kidneys during the filtration process.

3. **Sodium**: Sodium is an essential electrolyte that the kidneys filter and then reabsorb to maintain electrolyte balance. Thus, sodium is not removed but reabsorbed.

4. **Ammonia**: Ammonia is a toxic waste product that is primarily converted to urea in the liver. It is not removed directly from the blood by the kidneys.

### Step 3: Conclusion
Based on the analysis of the options, the only substance that is removed from the blood in the kidneys is **urea**.

### Final Answer
The correct answer is **urea**.

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