Final answer:
Urea is the solute that maintains the medullary interstitial fluid osmotic gradient in the kidneys, which is crucial for concentrating urine and managing body water levels.
Step-by-step explanation:
The solute that maintains the medullary interstitial fluid osmotic gradient is urea. This is vital for the kidney's ability to concentrate urine and excrete waste efficiently while retaining necessary water for body hydration. In the collecting ducts of the kidney, urea is actively pumped into the medulla, increasing the osmolarity of the interstitial fluid, which in turn helps retain water in the body. Sodium ions also contribute to the gradient through their active recovery from the ascending loop of Henle, but it is the urea that significantly accumulates in the medullary interstitial fluid to create a high osmotic environment.