Final answer:
The renal vein carries the purest blood with the lowest concentration of nitrogenous waste as it is responsible for transporting filtered blood from the kidneys back to the circulatory system.
Step-by-step explanation:
The blood vessel with the lowest concentration of nitrogenous waste (purest blood) would be the renal vein. The kidneys are responsible for filtering urea and other waste products from the blood, thus the blood exiting the kidneys through the renal vein has had most of the nitrogenous wastes removed. The process involves blood entering the kidney through the renal artery, being processed in nephrons where filtration and reabsorption occur, and ultimately the purified blood exits the kidney through the renal vein.
Within the nephrons, waste and excess substances are separated from the blood, converting these into urine. This urine is then collected in the ureters and stored in the urinary bladder. Consequently, the renal vein carries blood that is relatively free of nitrogenous waste back into the circulatory system.