In the loop of Henle, the countercurrent multiplier creates a hypertonic medullary interstitium. The single effect achieved at each level of the thick ascending limb is:

• A Active reabsorption of NaCl (via NKCC2) without water (impermeable to water), creating a 200 mOsm gradient between tubular fluid and adjacent interstitium at each level ✓
• B Passive water reabsorption from the thin descending limb adding solute-free water to the interstitium
• C Urea recycling from the inner medullary collecting duct into the interstitium, providing 50% of the osmotic gradient
• D Active H+ secretion in the thick ascending limb generating a pH gradient that drives NaCl transport

Explanation

The countercurrent multiplier mechanism depends on the thick ascending limb (TAL) being impermeable to water while actively transporting NaCl out via the apical NKCC2 (Na-K-2Cl cotransporter, blocked by furosemide). At each horizontal level, this active transport creates a transverse (single effect) osmotic gradient of ~200 mOsm between the tubular fluid and the adjacent interstitium. The countercurrent flow arrangement multiplies this single effect along the length of the loop, building up the corticomedullary osmotic gradient from ~300 mOsm at the cortex to ~1200 mOsm at the papillary tip. Urea recycling (option C) contributes to the inner medullary gradient but is a separate amplifying mechanism; water reabsorption from the thin descending limb (option B) concentrates the tubular fluid by osmosis but is not the 'single effect'. Option D is incorrect.

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

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Written and medically reviewed by the StethoPrep medical team.