Vitamins: vitamin A excess and retinoids: 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: vitamin A excess and retinoids

Sample item

A 19-year-old woman with severe nodulocystic acne is to start oral isotretinoin. She is counselled to use reliable contraception. Which mechanism underlies the teratogenic risk?

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

The answer and the reasoning

A. Excess retinoic acid acting through nuclear receptors disturbs Hox gene patterning in the embryo

Retinoic acid binds retinoic acid and retinoid X nuclear receptors and regulates gene expression, including Hox genes that pattern the craniofacial structures, heart, thymus and central nervous system. Excess exposure in the first trimester produces craniofacial, cardiac and CNS malformations. Pregnancy prevention programmes are mandatory with retinoids.

Option by option

  • A. Excess retinoic acid acting through nuclear receptors disturbs Hox gene patterning in the embryo (correct)

    Correct. Excess retinoic acid derails retinoid-receptor control of embryonic patterning genes.

  • B. Inhibition of vitamin K epoxide reductase in fetal liver

    Vitamin K epoxide reductase inhibition is the mechanism of warfarin embryopathy.

  • C. Inhibition of lysyl oxidase leading to collagen cross-linking defects

    Lysyl oxidase inhibition describes lathyrism or penicillamine-related collagen defects.

  • D. Folate antagonism through dihydrofolate reductase inhibition

    Folate antagonism characterises methotrexate embryopathy and neural tube defects with valproate.

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, Excess retinoic acid acting through nuclear receptors disturbs Hox gene patterning in the embryo. Retinoic acid binds retinoic acid and retinoid X nuclear receptors and regulates gene expression, including Hox genes that pattern the craniofacial structures, heart, thymus and central nervous system.

Why is "Inhibition of vitamin K epoxide reductase in fetal liver" not the answer?

Vitamin K epoxide reductase inhibition is the mechanism of warfarin embryopathy.

Why is "Inhibition of lysyl oxidase leading to collagen cross-linking defects" not the answer?

Lysyl oxidase inhibition describes lathyrism or penicillamine-related collagen defects.

Why is "Folate antagonism through dihydrofolate reductase inhibition" not the answer?

Folate antagonism characterises methotrexate embryopathy and neural tube defects with valproate.

What concept does this question test?

It tests Vitamins: vitamin A excess and retinoids in Biochemistry. Excess exposure in the first trimester produces craniofacial, cardiac and CNS malformations. Pregnancy prevention programmes are mandatory with retinoids.

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 Vitamins: vitamin A excess and retinoids?

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.