Vitamins and minerals: zinc and retinol-binding protein: 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 · Vitamins and minerals: zinc and retinol-binding protein

Sample item

A child with chronic diarrhoea and night blindness has a low serum retinol despite normal liver stores of vitamin A and a normal diet. His serum zinc is also low. Why does zinc deficiency reduce plasma retinol?

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

The answer and the reasoning

D. Zinc is needed for synthesis of retinol-binding protein, so retinol cannot be mobilised from the liver

Retinol is carried in plasma bound to retinol-binding protein made in the liver, and zinc is needed for its synthesis. In zinc deficiency the liver holds retinol but cannot release it, so plasma levels are low and night blindness may not respond to vitamin A alone. Zinc also forms part of retinol dehydrogenase in the retina.

Option by option

  • A. Zinc is needed to convert retinol to calcitriol

    Retinol is not converted to calcitriol, which comes from vitamin D.

  • B. Zinc is needed to absorb beta-carotene in the jejunum

    Beta-carotene absorption depends on dietary fat and bile, not on zinc.

  • C. Zinc deficiency destroys retinol by oxidation in plasma

    Zinc is not an antioxidant of plasma retinol, and deficiency does not destroy it.

  • D. Zinc is needed for synthesis of retinol-binding protein, so retinol cannot be mobilised from the liver (correct)

    Correct. Zinc is needed to make retinol-binding protein, so retinol stays trapped in the liver.

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, Zinc is needed for synthesis of retinol-binding protein, so retinol cannot be mobilised from the liver. Retinol is carried in plasma bound to retinol-binding protein made in the liver, and zinc is needed for its synthesis.

Why is "Zinc is needed to convert retinol to calcitriol" not the answer?

Retinol is not converted to calcitriol, which comes from vitamin D.

Why is "Zinc is needed to absorb beta-carotene in the jejunum" not the answer?

Beta-carotene absorption depends on dietary fat and bile, not on zinc.

Why is "Zinc deficiency destroys retinol by oxidation in plasma" not the answer?

Zinc is not an antioxidant of plasma retinol, and deficiency does not destroy it.

What concept does this question test?

It tests Vitamins and minerals: zinc and retinol-binding protein in Biochemistry. In zinc deficiency the liver holds retinol but cannot release it, so plasma levels are low and night blindness may not respond to vitamin A alone. Zinc also forms part of retinol dehydrogenase in the retina.

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 Vitamins and minerals: zinc and retinol-binding protein?

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.