It is the Hepatic vein.
Kidneys excrete urea, hence the blood vessels that exit from the kidneys have the least concentration of urea and other nitrogenous wastes.
Food contains proteins which are broken down into amino acids. The liver is functional in breaking down excess amino acids into urea. Therefore, the liver has the highest concentration of urea as it is the primary site for urea production. From the liver, the hepatic veins carry oxygen-depleted blood to the inferior vena cava. These veins emerge from the core vein of the liver lobule. The hepatic veins do not possess any valves.