Urine volume is increased in osmotic diuresis which may be experimentally produced by the intravenous administration of mannitol that is filtered in the glomerulus but not reabsorbed in the renal tubule. The following statements suggest some of the physiological mechanisms of osmotic diuresis.

A. In the proximal tubule, water reabsorption falls due to presence of mannitol in tubular fluid and concentration of Na+ is decreased in this fluid.
B. In the descending loop of Henle, reabsorption of water is increased as medullary hypertonicity is decreased in osmotic diuresis.
C. In the thin ascending loop of Henle, reabsorption of Na+ is increased as the concentration gradient for Na+ is decreased.
D. In the collecting duct, reabsorption of water is less because of decrease in osmotic gradient along the medullary pyramid in osmotic diuresis.

Which one of the following options represents the combination of all correct statements?

Correct option is D

Osmotic diuresis refers to increased urine output caused by substances (like mannitol) that are filtered by the glomeruli but not reabsorbed in the renal tubules.Statement A is correct: in the proximal tubule, mannitol increases the osmotic load of the tubular fluid, reducing passive water reabsorption and subsequently lowering Na⁺ concentration due to dilution.Statement D is also correct: in the collecting duct, water reabsorption is driven by the osmotic gradient between the tubular fluid and the hyperosmotic medullary interstitium. In osmotic diuresis, this gradient is blunted, reducing water reabsorption and contributing to increased urine output.

Correct Options:

• A (Proximal tubule water reabsorption falls):Mannitol increases the osmolarity of the tubular fluid, preventing water from following Na⁺ out of the tubule, thus reducing both water and sodium reabsorption in the proximal tubule.
  

• D (Reduced water reabsorption in the collecting duct):The medullary interstitium becomes less hypertonic due to washout of solutes, weakening the driving force for water reabsorption via aquaporins under ADH influence.

Incorrect Options:

• B (Water reabsorption increases in descending limb):Incorrect. Osmotic diuresis reduces medullary hypertonicity, which diminishes water reabsorption in the descending limb rather than increasing it. The driving force for water to move out of the descending limb is reduced.

• C (Na⁺ reabsorption increases in thin ascending limb):Incorrect. The thin ascending limb is impermeable to water, and active Na⁺ reabsorption occurs primarily in the thick ascending limb. In osmotic diuresis, medullary solute washout reduces Na⁺ reabsorption efficiency, not increases it.