To solve this problem, we need to determine which blood vessel in mammals carries the largest amount of urea. Let's analyze each option:$\newline \bullet$Renal vein:$\newline$ - The renal vein carries blood away from the kidneys after filtration.$\newline$ - Since the kidneys filter out urea, the renal vein would have a lower concentration of urea compared to the blood entering the kidneys.$\newline \bullet$Dorsal aorta:$\newline$ - The dorsal aorta distributes oxygenated blood from the heart to the rest of the body.$\newline$ - It does not specifically carry blood with high urea content.$\newline \bullet$Hepatic vein:$\newline$ - The hepatic vein carries blood away from the liver.$\newline$ - The liver is responsible for converting ammonia into urea, which is then released into the bloodstream.$\newline$ - Therefore, the hepatic vein carries blood with a high concentration of urea.$\newline \bullet$Hepatic portal vein:$\newline$ - The hepatic portal vein carries nutrient-rich blood from the digestive tract to the liver.$\newline$ - It does not carry blood with a high concentration of urea.$\newline$Based on this analysis, the blood vessel that would normally carry the largest amount of urea is the hepatic vein.$\newline$Therefore, the correct option is Option 3: Hepatic vein.