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The proximal tubules reabsorb >60% of the water filtered by the glomeruli, regardless of the patient's hydration status.  This water is reabsorbed isosmotically with solutes (eg, Na⁺, Cl^-, glucose); no concentration or dilution of urine occurs in this segment.

In the dehydrated state, plasma osmolarity increases, stimulating osmoreceptors in the anterior hypothalamus.  This triggers increased antidiuretic hormone (ADH) synthesis and release into the circulation.  ADH then acts on the kidney to improve the water permeability of the collecting ducts, allowing production of maximally concentrated urine (osmolarity of 1200 mOsm/L).  This increase in urine concentration with water deprivation reflects that the kidneys are functioning properly to conserve water.  However, without eventual fluid replenishment, dehydration will ultimately progress to death because even at maximum resorptive capacity, the kidney still produces approximately 0.5 L of urine per day.

(Choice C)  Urine concentration increases in the water-permeable descending loop of Henle due to the increasing osmolarity of the corticopapillary gradient in the renal interstitium; this segment normally reabsorbs about 20% of the filtered water volume.

(Choices A, B, and D)  In the dehydrated state, ADH promotes aquaporin (water channel) insertion into the apical membranes of the principal cells lining the late distal tubules and collecting ducts.  Up to 20% of the original filtered volume of water can be reabsorbed here, allowing >99% of filtered water to be resorbed by the nephron during dehydration.  No water is reabsorbed in these segments in the overhydrated state.

Educational objective:
Regardless of the patient's hydration status, the majority of water reabsorption in the nephron occurs in the proximal tubule passively with the reabsorption of solutes.