Respiratory system: airway resistance: 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 · Respiratory system: airway resistance

Sample item

During an asthma attack, bronchial smooth muscle contraction halves the radius of a medium airway. Assuming laminar flow, by what factor does the resistance of that airway change?

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

The answer and the reasoning

A. Increases 16-fold

In Poiseuille's law, resistance is inversely proportional to the fourth power of the radius. Halving the radius multiplies resistance by 2 to the power 4, which is 16. Small changes in bronchial calibre therefore have large effects on airflow, which is why bronchodilators are so effective.

Option by option

  • A. Increases 16-fold (correct)

    Resistance is proportional to 1/r4, so half the radius gives 2 x 2 x 2 x 2 = 16 times the resistance.

  • B. Increases 8-fold

    An 8-fold change would follow if resistance depended on 1/r3.

  • C. Increases 4-fold

    A 4-fold change would follow if resistance depended on 1/r2.

  • D. Increases 2-fold

    A 2-fold change would follow if resistance depended on 1/r, which is not the case.

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, Increases 16-fold. In Poiseuille's law, resistance is inversely proportional to the fourth power of the radius.

Why is "Increases 8-fold" not the answer?

An 8-fold change would follow if resistance depended on 1/r3.

Why is "Increases 4-fold" not the answer?

A 4-fold change would follow if resistance depended on 1/r2.

Why is "Increases 2-fold" not the answer?

A 2-fold change would follow if resistance depended on 1/r, which is not the case.

What concept does this question test?

It tests Respiratory system: airway resistance in Physiology. Halving the radius multiplies resistance by 2 to the power 4, which is 16. Small changes in bronchial calibre therefore have large effects on airflow, which is why bronchodilators are so effective.

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 Respiratory system: airway resistance?

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.