Respiratory system: diffusing capacity: 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: diffusing capacity

Sample item

A 45-year-old woman has exertional breathlessness. Spirometry and lung volumes are normal, but the diffusing capacity for carbon monoxide is markedly reduced. Which explanation fits best?

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

The answer and the reasoning

D. Loss of pulmonary capillary blood volume from pulmonary vascular disease

DLCO depends on the surface area and blood volume of the alveolar-capillary interface. With normal airways and volumes, a low DLCO points to pulmonary vascular disease or anaemia, where capillary blood volume is reduced. Asthma and chest wall causes leave DLCO normal or high.

Option by option

  • A. Airflow obstruction from asthma

    Asthma causes airflow obstruction on spirometry and usually leaves DLCO normal.

  • B. Obesity-related chest wall restriction

    Obesity restricts the chest wall and lowers volumes, and DLCO is usually normal.

  • C. Weakness of respiratory muscles

    Respiratory muscle weakness reduces volumes and pressures but not gas transfer across the membrane.

  • D. Loss of pulmonary capillary blood volume from pulmonary vascular disease (correct)

    Correct. Reduced capillary bed lowers gas transfer despite normal mechanics.

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 D, Loss of pulmonary capillary blood volume from pulmonary vascular disease. DLCO depends on the surface area and blood volume of the alveolar-capillary interface.

Why is "Airflow obstruction from asthma" not the answer?

Asthma causes airflow obstruction on spirometry and usually leaves DLCO normal.

Why is "Obesity-related chest wall restriction" not the answer?

Obesity restricts the chest wall and lowers volumes, and DLCO is usually normal.

Why is "Weakness of respiratory muscles" not the answer?

Respiratory muscle weakness reduces volumes and pressures but not gas transfer across the membrane.

What concept does this question test?

It tests Respiratory system: diffusing capacity in Physiology. With normal airways and volumes, a low DLCO points to pulmonary vascular disease or anaemia, where capillary blood volume is reduced. Asthma and chest wall causes leave DLCO normal or high.

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 Respiratory system: diffusing capacity?

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.