Final answer:
The ascending loop of Henle has the lowest water permeability in a normally functioning kidney, as it actively transports ions without allowing water to follow, thus diluting the filtrate.
Step-by-step explanation:
In the normally functioning kidney, the part of the tubule that has the lowest permeability to water under all conditions is the ascending loop of Henle. Unlike the descending limb, which is permeable to water allowing the reabsorption of water into the surrounding interstitial fluid, the ascending limb is impermeable to water. It actively transports Na+ and Cl- ions out of the filtrate, leaving the water behind, making the filtrate more dilute as it travels up the limb. The countercurrent multiplier mechanism involving the loop of Henle is essential in creating the osmotic gradient needed for the concentration of urine.