Progressive water reabsorption in kidney tubules reduces fluid volume

Steps:

• Glomerulus filters large volume of fluid into Bowman's capsule (tube X, ~180 L/day).
• Proximal tubule reabsorbs ~65% of water, so fluid entering loop of Henle (tube Y) is slightly less (~60 L/day).
• Further reabsorption in loop, distal tubule, and collecting duct (tube Z) reduces volume to ~1-2 L/day of urine.
• Compare daily volumes: Y slightly below X, but much above Z.

Why D is correct:

• Tubular reabsorption selectively removes water along the nephron, causing minimal initial reduction (X to Y) but major later reduction (Y to Z), per the definition of nephron function in urine concentration.

Why the others are wrong:

• A: Reabsorption ensures volume in Y is less than in X, not greater.
• B: Y is less than X but only slightly so, not significantly as implied.
• C: Y is only slightly less than X, not much less; it's much greater than Z.

Final answer: D