What is the therapeutic action of loop diuretics?

• A. They block sodium reabsorption in the ascending loop and dilate renal vessels.
• B. They increase glucose reabsorption in the proximal tubule.
• C. They increase potassium retention in the distal tubule.
• D. They inhibit aldosterone secretion.

Answer: (A)

Loop diuretics work by blocking the Na-K-2Cl cotransporter in the ascending loop of Henle, so sodium (and chloride) reabsorption is inhibited. Water follows, producing rapid diuresis. They also cause dilation of the renal vasculature, which lowers renal vascular resistance and can improve renal perfusion, aiding diuresis. Because more sodium reaches the distal nephron, potassium is excreted there, leading to potential hypokalemia. This mechanism explains their effectiveness in edema and hypertension, and why electrolyte monitoring is important. The other options don’t fit: increasing glucose reabsorption isn't how loop diuretics act; they promote potassium loss rather than retention; and they do not inhibit aldosterone secretion (a action of potassium-sparing diuretics).