**Core Concept**
Aldosterone is a mineralocorticoid hormone that regulates sodium and potassium balance by increasing sodium reabsorption and potassium excretion in the kidneys. Its primary site of action is in the distal nephron, specifically the collecting duct, where it enhances sodium reabsorption via epithelial sodium channels (ENaC).

**Why the Correct Answer is Right**
Aldosterone binds to mineralocorticoid receptors in the principal cells of the collecting duct, leading to increased expression of ENaC channels. This enhances sodium reabsorption and water retention, promoting volume expansion and blood pressure regulation. The hormone also stimulates potassium excretion and hydrogen ion secretion. This action occurs in the late distal tubule and collecting duct, not in earlier segments.

**Why Each Wrong Option is Incorrect**
Option B: The glomerulus is the site of filtration, not hormone action. Aldosterone does not act here.
Option C: The proximal convoluted tubule reabsorbs glucose, amino acids, and water but is not a site of aldosterone action.
Option D: The loop of Henle regulates osmolarity via active sodium reabsorption, primarily mediated by thiazide diuretics and the nephron’s countercurrent system, not aldosterone.

**Clinical Pearl / High-Yield Fact**
Remember: "Aldosterone acts in the **collecting duct** — not the loop of Henle or proximal tubule. It increases sodium reabsorption and potassium excretion, making it key in managing hypertension and hypokalemia."

✓ Correct Answer: A. Collecting duct