General physiology: Fick's law of diffusion: practice question

A Physiology 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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Physiology · General physiology: Fick's law of diffusion

Sample item

In a patient with advanced pulmonary fibrosis and emphysematous change, the blood-gas barrier is twice as thick and the effective surface area is half of normal. Other factors being equal, the diffusion rate of a gas across the barrier is:

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

The answer and the reasoning

A. One quarter of normal

By Fick's law, diffusion rate is proportional to surface area and partial pressure difference, and inversely proportional to thickness. Rate = (0.5 x area) / (2 x thickness) = 0.25 of normal. Both changes act multiplicatively.

Option by option

  • A. One quarter of normal (correct)

    Correct. Halving the area and doubling the thickness each halve the rate, giving a quarter.

  • B. One half of normal

    This accounts for only one of the two changes.

  • C. Unchanged

    The two changes do not offset each other; both reduce diffusion.

  • D. One eighth of normal

    Thickness enters as a simple inverse, not squared or cubed, so the result is not one eighth.

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 Physiology question

What is the answer to this Physiology question?

The best answer is A, One quarter of normal. By Fick's law, diffusion rate is proportional to surface area and partial pressure difference, and inversely proportional to thickness.

Why is "One half of normal" not the answer?

This accounts for only one of the two changes.

Why is "Unchanged" not the answer?

The two changes do not offset each other; both reduce diffusion.

Why is "One eighth of normal" not the answer?

Thickness enters as a simple inverse, not squared or cubed, so the result is not one eighth.

What concept does this question test?

It tests General physiology: Fick's law of diffusion in Physiology. Rate = (0.5 x area) / (2 x thickness) = 0.25 of normal. Both changes act multiplicatively.

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 General physiology: Fick's law of diffusion?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.