Which of the following statements about the hormone aldosterone is NOT correct regarding its site of production and action in the kidney?

Introduction / Context:
Aldosterone is an important mineralocorticoid hormone that plays a key role in regulating sodium, potassium, and water balance, and therefore blood pressure. It acts mainly on specific segments of the kidney tubules. Exam questions often mix accurate and inaccurate details about where aldosterone is produced and exactly how it works, and ask you to pick the statement that is NOT correct. That is the case in this question.

Given Data / Assumptions:

• Aldosterone is a hormone involved in sodium reabsorption and potassium secretion in the kidney.

• It is known to be produced in the adrenal cortex.

• The options describe its site of production, stimuli for secretion, and cellular mechanism of action.

• You must identify the one statement that does not correctly describe aldosterone physiology.

Concept / Approach:
Aldosterone is synthesized in the zona glomerulosa of the adrenal cortex. Its secretion increases when plasma sodium is low, plasma potassium is high, or when the renin–angiotensin system is activated. In the kidney, aldosterone acts mainly on principal cells of the late distal tubule and collecting duct, not primarily on proximal tubule cells. It increases sodium reabsorption and potassium secretion by increasing the number and activity of sodium channels on the luminal side and Na+-K+ ATPase pumps on the basolateral side of these cells. A statement that places Na+-K+ ATPase pumps in the luminal membrane of the proximal tubule as a direct aldosterone effect is therefore inaccurate.

Step-by-Step Solution:
Step 1: Check statement B, which says that aldosterone is produced in the adrenal cortex. This is true; more precisely, it is made in the zona glomerulosa layer. Step 2: Examine statement C, which claims that aldosterone secretion is stimulated by decreased plasma sodium and increased plasma potassium. This is largely correct; high potassium and low sodium, along with angiotensin II, are major stimulants. Step 3: Focus on statement A. It states that aldosterone increases sodium reabsorption by increasing Na+-K+ ATPase pumps in the luminal membrane of the proximal tubule. Step 4: Recall that Na+-K+ ATPase pumps are primarily located on the basolateral membrane, not the luminal membrane, of tubular cells. Step 5: Also recall that the main site of aldosterone action is the late distal tubule and collecting duct, not primarily the proximal tubule. Step 6: Therefore, statement A misplaces both the membrane side and the nephron segment where aldosterone acts most strongly and is thus not correct. Step 7: Since D says all the statements are correct, and we have already found one incorrect statement, option D must also be wrong as a description, leaving A as the specific incorrect statement the question is asking for.

Verification / Alternative check:
Renal physiology diagrams show aldosterone targeting principal cells in the cortical collecting ducts. The mechanism involves increased insertion of epithelial sodium channels on the luminal membrane and increased Na+-K+ ATPase activity on the basolateral membrane. Standard texts are clear that the proximal tubule reabsorbs a large fraction of filtered sodium but is not the primary segment under aldosterone control. These references reinforce that the description given in statement A conflicts with established physiology, while the descriptions of production site and stimuli in statements B and C are acceptable simplifications.

Why Other Options Are Wrong:

Option B: Correctly states that aldosterone comes from the adrenal cortex, so it is not the incorrect statement.

Option C: Correctly captures major stimuli for aldosterone release, so it is also not the incorrect statement.

Option D: Claims that all statements are correct, which is false because statement A is not correct.

Common Pitfalls:
A frequent mistake is to assume that all kidney segments are regulated by the same hormones or to ignore the difference between proximal and distal tubule segments. Another pitfall is confusion about luminal versus basolateral membrane and where Na+-K+ ATPase is located. Remember that aldosterone acts mainly on the distal nephron, with Na+-K+ ATPase located basolaterally and sodium channels on the luminal side. Any statement that shifts this mechanism to the proximal tubule or to the luminal membrane for the ATPase should be treated with caution.

Final Answer:
The statement that is NOT correct is that aldosterone increases sodium reabsorption by increasing Na+-K+ ATPase pumps in the luminal membrane of the proximal tubule.