Countercurrent Mechanism

In the countercurrent multiplier system of the loop of Henle, which segment is responsible for active transport of sodium and chloride without water reabsorption?

A 52-year-old woman with chronic kidney disease (CKD stage 3b, eGFR 38 mL/min/1.73m²) and hypertension is evaluated for persistent polyuria and nocturia despite adequate blood pressure control. Her serum sodium is 142 mEq/L (normal 135–145), serum osmolality is 298 mOsm/kg, and random urine osmolality is 280 mOsm/kg (inappropriately dilute). Renal ultrasound shows normal-sized kidneys with mild cortical thinning. Which aspect of the countercurrent mechanism is most likely impaired in this patient, and what is the pathophysiological consequence?

Which segment of the renal tubule is the commonest site of the countercurrent multiplier mechanism?

Regarding the countercurrent multiplier system in the loop of Henle, all of the following statements are correct EXCEPT:

A 35-year-old woman with chronic pyelonephritis and recurrent urinary tract infections is found to have a urine osmolality of 280 mOsm/kg despite a serum osmolality of 305 mOsm/kg. The attending physician suspects chronic tubulointerstitial damage affecting the countercurrent system. Which investigation would most specifically confirm loss of the medullary osmotic gradient?