**Core Concept**
Loop diuretics are a class of medications that increase urine production by inhibiting the reabsorption of ions and water in the renal tubules. They act by blocking the sodium-potassium-chloride cotransporter (NKCC2) in the thick ascending limb of the loop of Henle.

**Why the Correct Answer is Right**
The correct answer, **C. Ascending loop**, is the site of action for loop diuretics. The thick ascending limb of the loop of Henle is responsible for reabsorbing approximately 25% of the sodium, potassium, and chloride ions that entered the tubule in the descending limb. By inhibiting the NKCC2 cotransporter, loop diuretics prevent the reabsorption of these ions, leading to their excretion in the urine. This results in increased urine production and a decrease in blood volume.

**Option A:** PCT (Proximal Convoluted Tubule) is incorrect because loop diuretics do not act on the PCT, where glucose, amino acids, and bicarbonate are reabsorbed. **Option B:** DCT (Distal Convoluted Tubule) is incorrect because loop diuretics do not act on the DCT, where calcium and magnesium are reabsorbed. **Option D:** Descending loop is incorrect because loop diuretics act on the ascending limb, not the descending limb, of the loop of Henle.

**Clinical Pearl / High-Yield Fact**
Loop diuretics are often used in patients with heart failure, liver cirrhosis, and nephrotic syndrome to reduce fluid overload and edema. They are also used in the treatment of hypertension and certain types of kidney disease.

**✓ Correct Answer: C. Ascending loop**