Which one of the following blood vessel in mammals contains least amount of urea `:`

Options

A. Hepatic portal vein
B. Hepatic vein
C. Dorsal aorta
D. Renal vein

Answer

To determine which blood vessel in mammals contains the least amount of urea, we can analyze the options provided:

1. **Hepatic Portal Vein**: This vessel carries blood from the intestines to the liver. Since it collects blood that has absorbed nutrients and waste products (including urea) from the intestines, it is likely to have a higher concentration of urea.

2. **Hepatic Vein**: This vein carries blood away from the liver to the heart. The liver processes urea, converting ammonia into urea, so the hepatic vein would contain urea, but the concentration might be lower than that in the hepatic portal vein.

3. **Renal Artery**: This artery supplies blood to the kidneys. The blood in the renal artery contains urea as it is coming from the systemic circulation, where urea is present due to metabolic processes.

4. **Renal Vein**: This vein carries blood away from the kidneys back to the heart. The kidneys filter the blood, removing waste products including urea. Therefore, the renal vein would have a lower concentration of urea compared to the renal artery.

Based on this analysis, the blood vessel that contains the least amount of urea is the **Renal Vein**.

### Step-by-Step Solution:
1. Identify the blood vessels involved: Hepatic Portal Vein, Hepatic Vein, Renal Artery, and Renal Vein.
2. Analyze the function of each vessel:
- Hepatic Portal Vein: Carries blood from intestines to liver (higher urea).
- Hepatic Vein: Carries blood from liver to heart (contains processed urea).
- Renal Artery: Supplies blood to kidneys (contains urea).
- Renal Vein: Carries filtered blood from kidneys (contains least urea).
3. Conclude that the Renal Vein contains the least amount of urea.

### Final Answer:
The blood vessel in mammals that contains the least amount of urea is the **Renal Vein**.