Nitrogenous waste is at its highest concentration in (A) plasma in the renal vein - HSC - SSCE Biology - Question 12 - 2012 - Paper 1

Nitrogenous wastes, such as urea, are produced as byproducts of protein metabolism. In the kidney, these wastes undergo filtration and are concentrated in the fluid within the collecting ducts. This is because, during the process of reabsorption, much of the water and essential solutes are reabsorbed back into the bloodstream, leaving behind a higher concentration of nitrogenous waste in the urine within the collecting ducts.