**Core Concept**
Potassium-sparing diuretics act by modulating the function of the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system to reduce potassium excretion in the urine. These diuretics are used to treat conditions such as heart failure and hypertension.

**Why the Correct Answer is Right**
Potassium-sparing diuretics primarily act by antagonizing the effects of aldosterone at the collecting ducts of the kidneys. Aldosterone is a hormone produced by the adrenal glands that promotes the reabsorption of sodium and water while increasing the excretion of potassium. By blocking the action of aldosterone, potassium-sparing diuretics decrease the amount of potassium excreted in the urine, thereby "sparring" potassium levels in the body. This is achieved through the inhibition of the sodium-potassium-chloride cotransporter (NKCC2) or the epithelial sodium channel (ENaC) in the collecting ducts.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because potassium-sparing diuretics do not primarily act at the level of the loop of Henle, where loop diuretics such as furosemide exert their effects.
* **Option B:** This option is incorrect because potassium-sparing diuretics do not primarily act at the level of the proximal convoluted tubule (PCT), where carbonic anhydrase inhibitors exert their effects.
* **Option C:** This option is incorrect because potassium-sparing diuretics do not primarily act at the level of the ascending limb of the loop of Henle, where thiazide diuretics exert their effects.

**Clinical Pearl / High-Yield Fact**
It is essential to note that potassium-sparing diuretics can lead to hyperkalemia as a side effect, especially when used in combination with other potassium-sparing agents or in patients with renal impairment. Therefore, monitoring of serum potassium levels is crucial when prescribing these medications.

**Correct Answer:** C. Collecting ducts.