The diagram given below represents the liver, kidney and some associated blood vessels. Identify the vessel from the labelled parts A-D in which the blood will contain the lowest concentration of urea?

The urea is formed by the urea-cycle in the liver, and not in the kidneys. The blood vessel coming out from the kidney will contain the lowest concentration of urea (although kidneys contain only a small amount of urea to maintain osmolarity). And the blood vessel coming out from the liver will contain the highest concentration of urea.