Respiratory system: diffusing capacity for carbon monoxide: 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 for carbon monoxide

Sample item

A 32-year-old man with haemoptysis, haematuria and normal spirometry has a diffusing capacity for carbon monoxide (DLCO) above the predicted value. What explains the raised DLCO?

AAn increase in alveolar surface area from emphysemaBHaemoglobin from blood within the alveoli takes up inhaled carbon monoxideCA reduced haemoglobin concentration in the bloodDA thickened alveolar membrane increases the rate of diffusion

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

The answer and the reasoning

B. Haemoglobin from blood within the alveoli takes up inhaled carbon monoxide

Extravascular red cells within the alveoli bind carbon monoxide avidly, so the measured uptake increases in pulmonary haemorrhage such as Goodpasture syndrome. DLCO is also raised in polycythaemia, exercise and left-to-right shunts, and is reduced in anaemia, emphysema and fibrosis.

Option by option

A. An increase in alveolar surface area from emphysema
Emphysema destroys surface area and lowers DLCO.
B. Haemoglobin from blood within the alveoli takes up inhaled carbon monoxide (correct)
Correct. Intra-alveolar blood adds extra carbon monoxide uptake.
C. A reduced haemoglobin concentration in the blood
Anaemia lowers DLCO, because there is less haemoglobin to bind carbon monoxide.
D. A thickened alveolar membrane increases the rate of diffusion
Membrane thickening slows diffusion and lowers DLCO.

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 B, Haemoglobin from blood within the alveoli takes up inhaled carbon monoxide. Extravascular red cells within the alveoli bind carbon monoxide avidly, so the measured uptake increases in pulmonary haemorrhage such as Goodpasture syndrome.

Why is "An increase in alveolar surface area from emphysema" not the answer?

Emphysema destroys surface area and lowers DLCO.

Why is "A reduced haemoglobin concentration in the blood" not the answer?

Anaemia lowers DLCO, because there is less haemoglobin to bind carbon monoxide.

Why is "A thickened alveolar membrane increases the rate of diffusion" not the answer?

Membrane thickening slows diffusion and lowers DLCO.

What concept does this question test?

It tests Respiratory system: diffusing capacity for carbon monoxide in Physiology. DLCO is also raised in polycythaemia, exercise and left-to-right shunts, and is reduced in anaemia, emphysema and fibrosis.

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 carbon monoxide?

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.