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MBBS QUESTION #10262
Question 1
Production of concentrated (hypertonic) urine depends on which key physiological mechanism operating in the loop of Henle and collecting duct?
  • The countercurrent multiplier system generating a hypertonic renal medullary interstitium, combined with ADH-regulated water permeability of the collecting duct✔️
  • Simple glomerular filtration alone, with no further tubular modification
  • Active water secretion by the distal convoluted tubule
  • Complete, fixed impermeability of the collecting duct to water under all conditions
Correct Answer Explanation
The loop of Henle's countercurrent multiplier establishes a progressively hypertonic medullary interstitium; in the presence of ADH, the collecting duct becomes permeable to water, which is then reabsorbed osmotically into this hypertonic interstitium, producing concentrated urine — impaired in both central and nephrogenic diabetes insipidus, though by different mechanisms.