Which statement about loop diuretics is true?

• A. They act on the distal tubule to promote potassium retention
• B. They are never used for edema
• C. They are not potent
• D. They act on the loop of Henle and cause loss of potassium, calcium, and magnesium

Answer: (D)

Loop diuretics act on the loop of Henle, specifically the thick ascending limb, where they inhibit the Na+-K+-2Cl− cotransporter. By blocking this transporter, they prevent reabsorption of sodium, chloride, and potassium, leading to powerful diuresis. The blockade also disrupts the lumen-positive potential that normally drives paracellular reabsorption of divalent cations, so calcium and magnesium are wasted in the urine. The net result is potent diuresis with losses of potassium, calcium, and magnesium, which is why this statement is true. These drugs are indeed used to treat edema, and they are among the most potent diuretics.