Which of the following statements about the hormone aldosterone is NOT correct?

Introduction / Context:
Aldosterone is a key mineralocorticoid hormone that helps regulate fluid balance, sodium and potassium levels, and blood pressure. It acts mainly on specific segments of the nephron in the kidney. This question asks you to identify which statement about aldosterone is not correct, so you must know both where it acts and how it changes transport proteins in renal tubular cells.

Given Data / Assumptions:

Aldosterone influences sodium and potassium handling by the kidney.
The nephron includes the proximal tubule, distal tubule, and collecting duct.
Transport proteins include Na plus channels and Na plus K plus ATPase pumps on different cell membranes.

Concept / Approach:
Aldosterone is produced in the zona glomerulosa of the adrenal cortex. It is released in response to signals such as high plasma potassium levels and the renin angiotensin system, often associated with low blood pressure or low sodium. Aldosterone acts mainly on the late distal tubule and collecting duct, not on the proximal tubule. In principal cells in these segments, it increases the number and activity of Na plus K plus ATPase pumps on the basolateral membrane and increases sodium channel numbers in the luminal (apical) membrane, which together enhance sodium reabsorption and potassium secretion. Any statement that places these actions in the wrong nephron segment or on the wrong membrane surface is not correct.

Step-by-Step Solution:
Step 1: Confirm that aldosterone is indeed synthesized in the adrenal cortex, making that statement correct. Step 2: Recall that aldosterone secretion is stimulated by high plasma potassium and by signals related to low blood volume or low sodium, so the statement about its stimuli is broadly correct. Step 3: Recognize that aldosterone acts mainly on the distal tubule and collecting duct to increase luminal sodium channels and basolateral Na plus K plus ATPase, so the statement describing increased luminal channels in these segments is appropriate. Step 4: Identify that the statement placing Na plus K plus ATPase pumps in the luminal membrane of the proximal tubule is incorrect, because these pumps are on the basolateral side and the major aldosterone action is not in the proximal tubule.

Verification / Alternative check:
Standard physiology diagrams show principal cells with luminal sodium channels that allow sodium entry from tubular fluid and basolateral Na plus K plus ATPase pumps that move sodium into the blood and bring potassium into the cells. Aldosterone enhances both elements in the distal nephron. The proximal tubule reabsorbs sodium largely in an aldosterone independent manner using different transporters. This supports the conclusion that the proximal tubule statement is wrong.

Why Other Options Are Wrong:
Stimulated by low sodium and high potassium: This captures the main regulatory triggers of aldosterone, so it is acceptable as a correct statement.
Produced in adrenal cortex: This is a textbook fact and is definitely correct.
Increases passive sodium channels in distal tubule and collecting duct: This correctly describes part of aldosterone action in the appropriate nephron segments.

Common Pitfalls:
Students often confuse which nephron segments are hormone sensitive and may think all sodium reabsorption is equally under aldosterone control. Another confusion is between apical (luminal) and basolateral membranes. Remember that Na plus K plus ATPase is a basolateral pump in most epithelial cells, including kidney tubule cells. Carefully reading the location details in each statement is crucial for identifying the incorrect one.

Final Answer:
The incorrect statement is that aldosterone increases sodium reabsorption by increasing Na plus K plus ATPase pumps in the luminal membrane of the proximal tubule, which misstates both the site of action and membrane location of the pump.