Cardiac mechanics: Laplace law: 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 · Cardiac mechanics: Laplace law

Sample item

In a patient with dilated cardiomyopathy, the left ventricular radius increases from 2 cm to 3 cm. If intracavitary pressure and wall thickness stay the same, what happens to wall stress according to the law of Laplace?

AIt is unchangedBIt falls by 33%CIt doublesDIt increases by 50%

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

The answer and the reasoning

D. It increases by 50%

Wall stress is proportional to pressure multiplied by radius and divided by wall thickness (P x r / 2h). With pressure and thickness constant, stress rises in direct proportion to radius: 3 / 2 = 1.5, a 50% increase. A dilated ventricle therefore needs more wall stress, and more oxygen, to generate the same pressure.

Option by option

A. It is unchanged
Stress would be unchanged only if radius did not affect it, which contradicts the Laplace relationship.
B. It falls by 33%
A fall of 33% would result if radius decreased to two-thirds, the reverse of this situation.
C. It doubles
Doubling would need the radius to double, from 2 cm to 4 cm.
D. It increases by 50% (correct)
Correct. Stress is proportional to radius, so a 1.5-fold larger radius gives a 50% rise.

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.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is D, It increases by 50%. Wall stress is proportional to pressure multiplied by radius and divided by wall thickness (P x r / 2h).

Why is "It is unchanged" not the answer?

Stress would be unchanged only if radius did not affect it, which contradicts the Laplace relationship.

Why is "It falls by 33%" not the answer?

A fall of 33% would result if radius decreased to two-thirds, the reverse of this situation.

Why is "It doubles" not the answer?

Doubling would need the radius to double, from 2 cm to 4 cm.

What concept does this question test?

It tests Cardiac mechanics: Laplace law in Physiology. With pressure and thickness constant, stress rises in direct proportion to radius: 3 / 2 = 1.5, a 50% increase. A dilated ventricle therefore needs more wall stress, and more oxygen, to generate the same pressure.

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 Cardiac mechanics: Laplace law?

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.