At the end of the proximal convoluted tubule, the tubular fluid is iso-osmotic to plasma (approximately 300 mOsm/kg) despite reabsorption of two-thirds of the filtered water and solutes. This occurs because:

• A The proximal tubule is impermeable to water
• B Water reabsorption follows solute reabsorption isotonically due to high water permeability ✓
• C Aquaporin-3 channels prevent water movement
• D Active water transport maintains osmotic equilibrium

Explanation

The proximal tubule has constitutively high water permeability via AQP1. Water follows reabsorbed solutes passively, so tubular fluid remains iso-osmotic throughout. There is no active water transport in the nephron. AQP1 (not AQP3) is the proximal tubule aquaporin.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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