The correct answer is option 3, the hepatic vein. Urea is a waste product formed in the liver when proteins are metabolized. It is then transported to the kidneys to be excreted in urine. The highest concentration of urea is found in the hepatic vein because this vein carries blood away from the liver, which is the site of urea synthesis. As blood passes through the liver, urea is produced and accumulates in the hepatic vein before being distributed to the rest of the body. Option 1, the hepatic portal vein, is incorrect because this vein carries blood to the liver, not away from it. Option 2, the dorsal aorta, is incorrect because it is a major artery that carries oxygenated blood away from the heart, not waste products like urea. Option 4, the renal vein, is incorrect because the urea concentration is highest in the hepatic vein before it reaches the kidneys.