Q:

Which blood vessel carries the most impure blood?

There is a question in my Biology textbook:

"Which blood vessel in the human body carries most impure blood after
  few hours of meal."

The answer is given as Renal artery.
But I think the answer should be Hepatic Vein, because the liver produces urea and other waste materials and then it pours it all in the right ventricle of the heart for oxygenation. And then the heart distributes the blood to various parts of the body. So the impure blood brought by the Hepatic Vein and other blood vessels gets distributed through the aorta. This will automatically imply that the Renal Artery will contain lesser impure blood than the Hepatic Vein.
Impure blood is the blood with urea, uric acid and other components of urine.
It would be good if you give your opinion and also a reason for it.

A:

It's the renal artery because it will contain everything from the liver as you describe plus all waste accumulated on the way from the liver to the kidney. Here it gets filtered, so there cannot be a place after the kidney that is less "pure".

