Question

Hyperosmolarity of the renal medulla is most importantly due to:

• A. Potassium
• B. Sodium
• C. Glucose
• D. Chloride

Hint:

The countercurrent mechanism deposits mainly NaCl; sodium is the dominant solute.

Answer 1

The Correct Option is B

Step 1: The high osmolarity of the renal medullary interstitium is created by the countercurrent mechanism. The thick ascending limb of the loop of Henle is impermeable to water but actively pumps NaCl out into the interstitium, while the thin descending limb is permeable to water but not solute.

Step 2: The single most important contributor is the active transport of sodium, with co-transport of potassium and chloride, out of the thick ascending limb. Among all the deposited solutes, NaCl is the dominant osmotically active species responsible for medullary hyperosmolarity.

Step 3: Urea contributes secondarily through urea recycling in the inner medulla, while glucose plays essentially no role. Of the four options, sodium (as NaCl) is the chief cause, so the correct single answer is sodium.

Step 4: Although potassium and chloride are co-transported with sodium, the question asks for the most important factor; the explanation itself states that NaCl is the most important, making sodium the best single answer.