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 need to analyze the function of different blood vessels involved in the excretory system, particularly focusing on the nephron's role in filtering blood.

### Step-by-Step Solution:

1. **Understand the Structure of the Nephron**:
- The nephron is the functional unit of the kidney, responsible for filtering blood and forming urine. It consists of several parts, including the glomerulus, Bowman's capsule, proximal tubule, loop of Henle, distal tubule, and collecting duct.

2. **Identify Key Blood Vessels**:
- The key blood vessels involved in this process are the afferent arteriole, efferent arteriole, renal vein, and the hepatic portal vein.

3. **Function of the Afferent and Efferent Arterioles**:
- The afferent arteriole brings blood into the glomerulus, where filtration occurs. This blood contains waste products, including urea.
- The efferent arteriole carries blood away from the glomerulus after filtration. This blood has already lost some waste products, including urea, as they are filtered into the nephron tubules.

4. **Role of the Renal Vein**:
- After the filtration and reabsorption processes in the nephron, the renal vein carries the filtered blood away from the kidney. This blood has the least amount of urea because most of the urea and other waste products have been excreted into the urine.

5. **Comparison with Other Blood Vessels**:
- The hepatic portal vein carries blood from the digestive organs to the liver, which may contain nutrients and waste products.
- The dorsal aorta carries oxygenated blood from the heart to the body and does not specifically filter out urea.
- The hepatic vein carries blood away from the liver, which may still contain some waste products.

6. **Conclusion**:
- Among the options provided, the renal vein contains the least amount of urea because it carries blood that has been filtered and has had most of its waste products removed.

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