Mineral metabolism: 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

Sample item

A man with advanced chronic kidney disease has low serum calcium, high phosphate and raised parathyroid hormone. Which biochemical defect most directly lowers his calcium absorption?

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

The answer and the reasoning

D. Reduced renal 1-alpha hydroxylase activity, lowering calcitriol

The kidney converts 25-hydroxyvitamin D into the active calcitriol using 1-alpha hydroxylase. In advanced kidney disease this enzyme falls and phosphate retention further suppresses it, so intestinal calcium absorption drops and secondary hyperparathyroidism develops.

Option by option

  • A. Reduced hepatic 25-hydroxylation of vitamin D

    25-hydroxylation occurs in the liver, which is intact in this man.

  • B. Reduced skin synthesis of cholecalciferol

    Skin synthesis of cholecalciferol is unaffected by kidney disease itself.

  • C. Excess intestinal calbindin

    Calbindin facilitates calcium transport in the gut; excess would increase absorption rather than reduce it.

  • D. Reduced renal 1-alpha hydroxylase activity, lowering calcitriol (correct)

    Correct. Failing kidneys cannot make enough calcitriol, so gut calcium absorption falls.

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 D, Reduced renal 1-alpha hydroxylase activity, lowering calcitriol. The kidney converts 25-hydroxyvitamin D into the active calcitriol using 1-alpha hydroxylase.

Why is "Reduced hepatic 25-hydroxylation of vitamin D" not the answer?

25-hydroxylation occurs in the liver, which is intact in this man.

Why is "Reduced skin synthesis of cholecalciferol" not the answer?

Skin synthesis of cholecalciferol is unaffected by kidney disease itself.

Why is "Excess intestinal calbindin" not the answer?

Calbindin facilitates calcium transport in the gut; excess would increase absorption rather than reduce it.

What concept does this question test?

It tests Mineral metabolism in Biochemistry. In advanced kidney disease this enzyme falls and phosphate retention further suppresses it, so intestinal calcium absorption drops and secondary hyperparathyroidism develops.

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?

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.