Which nephron segment is impermeable to water but actively reabsorbs ions, contributing to the counter-current multiplier system and concentrating the medullary interstitium?
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Correct answer: Thick ascending limb of loop of Henle
The thick ascending limb is impermeable to water but reabsorbs Na⁺, K⁺, and Cl⁻ via NKCC2; by doing so, it dilutes tubular fluid and contributes to the hyperosmolar medullary gradient essential for urine concentration.
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