In the 'Countercurrent' concept for chloride, which ion is listed?

• A. Sodium
• B. Chloride
• C. Potassium
• D. Bicarbonate

Answer: (C)

The question taps into the countercurrent mechanism in the thick ascending limb of the loop of Henle, where sodium, potassium, and chloride are reabsorbed together by the Na+-K+-2Cl− cotransporter. This transporter relies on potassium as the counterion that accompanies chloride as it moves from the lumen into the tubular cells, a process that helps establish the medullary osmotic gradient essential for urine concentration. Because of this co-transport arrangement, potassium is the ion listed alongside chloride in this countercurrent concept. Sodium and bicarbonate aren’t the primary partners in this specific transporter, and chloride isn’t by itself the partner ion in this context.