Respiratory system: shunt versus V/Q mismatch: 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: shunt versus V/Q mismatch

Sample item

A patient with lobar pneumonia has PaO2 of 55 mmHg on room air. After breathing 100 percent oxygen for 20 minutes, the PaO2 rises only to 70 mmHg. Which mechanism explains the poor response?

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

The answer and the reasoning

C. Right-to-left shunt through fluid-filled alveoli

In a shunt, blood passes unventilated alveoli and no extra oxygen can reach it, so oxygen therapy corrects hypoxaemia poorly. Hypoventilation, diffusion limitation and low V/Q respond to supplemental oxygen.

Option by option

  • A. Diffusion limitation

    Diffusion limitation is overcome by raising alveolar oxygen.

  • B. Pure hypoventilation

    Hypoventilation corrects with oxygen and gives a normal A-a gradient.

  • C. Right-to-left shunt through fluid-filled alveoli (correct)

    Correct. A shunt does not correct with oxygen since blood bypasses ventilated gas exchange.

  • D. Low ventilation to perfusion ratio only

    Low V/Q areas still receive some ventilation and improve with oxygen.

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 C, Right-to-left shunt through fluid-filled alveoli. In a shunt, blood passes unventilated alveoli and no extra oxygen can reach it, so oxygen therapy corrects hypoxaemia poorly.

Why is "Diffusion limitation" not the answer?

Diffusion limitation is overcome by raising alveolar oxygen.

Why is "Pure hypoventilation" not the answer?

Hypoventilation corrects with oxygen and gives a normal A-a gradient.

Why is "Low ventilation to perfusion ratio only" not the answer?

Low V/Q areas still receive some ventilation and improve with oxygen.

What concept does this question test?

It tests Respiratory system: shunt versus V/Q mismatch in Physiology. Hypoventilation, diffusion limitation and low V/Q respond to supplemental oxygen.

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: shunt versus V/Q mismatch?

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.