Mineral metabolism: hypomagnesaemia and calcium: practice question

A Biochemistry 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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Biochemistry · Mineral metabolism: hypomagnesaemia and calcium

Sample item

A 58-year-old woman on long-term proton pump inhibitor and a loop diuretic has tetany. Serum calcium is 7.0 mg/dL, potassium 3.0 mmol/L and PTH is inappropriately low, with serum magnesium 0.9 mg/dL. Calcium infusion gives only a transient benefit. Why is magnesium replacement required?

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

The answer and the reasoning

A. Hypomagnesaemia impairs PTH secretion and causes end-organ resistance to PTH

Magnesium is needed for release of PTH from the parathyroid and for PTH action at its receptor and adenylate cyclase in bone and kidney. Severe depletion therefore produces functional hypoparathyroidism and refractory hypocalcaemia and hypokalaemia. Correction needs intravenous or oral magnesium, after which calcium normalises.

Option by option

  • A. Hypomagnesaemia impairs PTH secretion and causes end-organ resistance to PTH (correct)

    Correct. Low magnesium blunts PTH secretion and its tissue effect, so calcium alone fails.

  • B. Hypomagnesaemia stimulates calcitonin release which lowers calcium

    Calcitonin is not stimulated by hypomagnesaemia.

  • C. Hypomagnesaemia promotes calcium deposition in bone

    Magnesium deficiency does not promote bone mineral deposition and would not explain refractory hypocalcaemia.

  • D. Hypomagnesaemia binds ionised calcium in plasma

    Magnesium does not chelate plasma calcium in this way.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Hypomagnesaemia impairs PTH secretion and causes end-organ resistance to PTH. Magnesium is needed for release of PTH from the parathyroid and for PTH action at its receptor and adenylate cyclase in bone and kidney.

Why is "Hypomagnesaemia stimulates calcitonin release which lowers calcium" not the answer?

Calcitonin is not stimulated by hypomagnesaemia.

Why is "Hypomagnesaemia promotes calcium deposition in bone" not the answer?

Magnesium deficiency does not promote bone mineral deposition and would not explain refractory hypocalcaemia.

Why is "Hypomagnesaemia binds ionised calcium in plasma" not the answer?

Magnesium does not chelate plasma calcium in this way.

What concept does this question test?

It tests Mineral metabolism: hypomagnesaemia and calcium in Biochemistry. Severe depletion therefore produces functional hypoparathyroidism and refractory hypocalcaemia and hypokalaemia. Correction needs intravenous or oral magnesium, after which calcium normalises.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Mineral metabolism: hypomagnesaemia and calcium?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.