Renal and acid–base: magnesium and calcium handling in the thick ascending limb: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Renal and acid–base: magnesium and calcium handling in the thick ascending limb

Sample item

A patient on long-term high-dose furosemide has increased urinary magnesium and calcium excretion. Which mechanism explains this?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Loss of the lumen-positive potential that drives paracellular divalent cation reabsorption

Potassium recycling through ROMK in the thick ascending limb makes the lumen positive relative to the interstitium, and this potential drives paracellular reabsorption of calcium and magnesium. Blocking NKCC2 with a loop diuretic abolishes the potential, so both ions are lost in the urine.

Option by option

  • A. Inhibition of PTH secretion by the diuretic

    PTH secretion is not inhibited by loop diuretics.

  • B. Loss of the lumen-positive potential that drives paracellular divalent cation reabsorption (correct)

    Correct. The lumen-positive potential that drives paracellular reabsorption is lost.

  • C. Increased reabsorption of sodium in the proximal tubule

    Loop diuretics do not increase proximal sodium reabsorption.

  • D. Direct inhibition of calcium channels in the distal tubule

    Loop diuretics act on the thick ascending limb, and thiazides, not loop diuretics, act on the distal tubule calcium handling.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is B, Loss of the lumen-positive potential that drives paracellular divalent cation reabsorption. Potassium recycling through ROMK in the thick ascending limb makes the lumen positive relative to the interstitium, and this potential drives paracellular reabsorption of calcium and magnesium.

Why is "Inhibition of PTH secretion by the diuretic" not the answer?

PTH secretion is not inhibited by loop diuretics.

Why is "Increased reabsorption of sodium in the proximal tubule" not the answer?

Loop diuretics do not increase proximal sodium reabsorption.

Why is "Direct inhibition of calcium channels in the distal tubule" not the answer?

Loop diuretics act on the thick ascending limb, and thiazides, not loop diuretics, act on the distal tubule calcium handling.

What concept does this question test?

It tests Renal and acid–base: magnesium and calcium handling in the thick ascending limb in Physiology. Blocking NKCC2 with a loop diuretic abolishes the potential, so both ions are lost in the urine.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Renal and acid–base: magnesium and calcium handling in the thick ascending limb?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.