In mammals, which blood vessel would normally carry the largest amount of urea?

The correct option is C Hepatic Vein. The hepatic portal vein leads from the small intestine to the liver. The hepatic vein carries blood away from the liver to the vena cava which transports it back to the heart. The hepatic vein has the highest concentration of urea. This is because urea is a by-product of amino acid catabolism, that occurs in the liver. Hence liver is the primary site for urea production. So, the correct answer is 'Hepatic Vein'.