Vitamins: visual cycle: 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: visual cycle

Sample item

In a dark-adapted rod, absorption of a photon by rhodopsin causes which primary photochemical change?

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

The answer and the reasoning

D. Isomerisation of 11-cis retinal to all-trans retinal

Rhodopsin contains 11-cis retinal bound to opsin. Light converts it to all-trans retinal, which alters opsin to metarhodopsin II and activates transducin, so cGMP phosphodiesterase lowers cGMP, channels close and the rod hyperpolarises. All-trans retinal is later recycled to the 11-cis form in the pigment epithelium.

Option by option

  • A. Phosphorylation of opsin by rhodopsin kinase

    Phosphorylation by rhodopsin kinase is a later step that terminates the signal.

  • B. Isomerisation of all-trans retinal to 11-cis retinal

    Regeneration of 11-cis retinal happens afterwards in the dark in the retinal pigment epithelium.

  • C. Oxidation of retinol to retinal

    Retinol to retinal is a dehydrogenase reaction in the recycling pathway and not the primary event.

  • D. Isomerisation of 11-cis retinal to all-trans retinal (correct)

    Light-driven cis to trans isomerisation is the primary event that triggers the phototransduction cascade.

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, Isomerisation of 11-cis retinal to all-trans retinal. Rhodopsin contains 11-cis retinal bound to opsin.

Why is "Phosphorylation of opsin by rhodopsin kinase" not the answer?

Phosphorylation by rhodopsin kinase is a later step that terminates the signal.

Why is "Isomerisation of all-trans retinal to 11-cis retinal" not the answer?

Regeneration of 11-cis retinal happens afterwards in the dark in the retinal pigment epithelium.

Why is "Oxidation of retinol to retinal" not the answer?

Retinol to retinal is a dehydrogenase reaction in the recycling pathway and not the primary event.

What concept does this question test?

It tests Vitamins: visual cycle in Biochemistry. Light converts it to all-trans retinal, which alters opsin to metarhodopsin II and activates transducin, so cGMP phosphodiesterase lowers cGMP, channels close and the rod hyperpolarises. All-trans retinal is later recycled to the 11-cis form in the pigment epithelium.

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: visual cycle?

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.