The concentration of urine to 1200 mOsm/kg in the collecting duct requires which of the following prerequisites?

• A Absence of ADH and a hypertonic medullary interstitium
• B Presence of ADH and a hypertonic medullary interstitium created by the countercurrent multiplier ✓
• C High aldosterone levels only
• D Increased GFR and high tubular flow rate

Explanation

Maximal urinary concentration requires both ADH (which inserts aquaporin-2 channels into the collecting duct apical membrane) and a pre-existing hypertonic medullary interstitium (up to 1200 mOsm/kg at the papilla). The hypertonic medulla is built by the countercurrent multiplier in the loop of Henle and is sustained by the countercurrent exchanger in the vasa recta. Without ADH, the collecting duct remains impermeable to water regardless of medullary tonicity.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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