Physiology: phototransduction: practice question

A Ophthalmology 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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Ophthalmology · Physiology: phototransduction

Sample item

In phototransduction, what is the immediate effect of light on a rod photoreceptor?

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

The answer and the reasoning

D. Fall in cGMP, closure of sodium channels and hyperpolarisation

Light isomerises 11-cis retinal to all-trans, activating transducin and phosphodiesterase, which hydrolyses cGMP. Sodium channels close and the cell hyperpolarises, reducing glutamate release. In the dark, high cGMP keeps photoreceptors depolarised.

Option by option

  • A. Opening of calcium channels and neurotransmitter release

    Neurotransmitter release is reduced in light, not increased.

  • B. Increase in dark current

    The dark current is reduced by light.

  • C. Rise in cGMP and depolarisation

    cGMP rises in the dark, not on exposure to light.

  • D. Fall in cGMP, closure of sodium channels and hyperpolarisation (correct)

    Correct. Light causes a cGMP fall and hyperpolarisation.

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 Ophthalmology question

What is the answer to this Ophthalmology question?

The best answer is D, Fall in cGMP, closure of sodium channels and hyperpolarisation. Light isomerises 11-cis retinal to all-trans, activating transducin and phosphodiesterase, which hydrolyses cGMP.

Why is "Opening of calcium channels and neurotransmitter release" not the answer?

Neurotransmitter release is reduced in light, not increased.

Why is "Increase in dark current" not the answer?

The dark current is reduced by light.

Why is "Rise in cGMP and depolarisation" not the answer?

cGMP rises in the dark, not on exposure to light.

What concept does this question test?

It tests Physiology: phototransduction in Ophthalmology. Sodium channels close and the cell hyperpolarises, reducing glutamate release. In the dark, high cGMP keeps photoreceptors depolarised.

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 Physiology: phototransduction?

Group the red eye by what to examine and what the pupil does. Then use the flashcards and the other Ophthalmology 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.