Respiratory system: single-breath nitrogen test: 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: single-breath nitrogen test

Sample item

A subject takes one breath of 100 percent oxygen and then exhales slowly while nitrogen concentration is recorded. The volume exhaled before nitrogen reaches its alveolar plateau is used to estimate which quantity?

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

The answer and the reasoning

C. Anatomical dead space

In the single-breath nitrogen method of Fowler, the first gas exhaled is pure oxygen from the conducting airways, with no nitrogen. The volume at the midpoint of the rise gives the anatomical dead space, about 150 mL. The slope of the plateau and the later upswing reflect uneven ventilation and closing volume.

Option by option

  • A. Closing volume

    Closing volume is found from the later phase IV upswing, not the initial rise.

  • B. Residual volume

    Residual volume needs a dilution method or plethysmography and is not given by this phase.

  • C. Anatomical dead space (correct)

    Correct. The nitrogen-free volume from the conducting airways equals anatomical dead space.

  • D. Physiological dead space

    Physiological dead space is found using the Bohr equation with arterial and expired CO2.

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, Anatomical dead space. In the single-breath nitrogen method of Fowler, the first gas exhaled is pure oxygen from the conducting airways, with no nitrogen.

Why is "Closing volume" not the answer?

Closing volume is found from the later phase IV upswing, not the initial rise.

Why is "Residual volume" not the answer?

Residual volume needs a dilution method or plethysmography and is not given by this phase.

Why is "Physiological dead space" not the answer?

Physiological dead space is found using the Bohr equation with arterial and expired CO2.

What concept does this question test?

It tests Respiratory system: single-breath nitrogen test in Physiology. The volume at the midpoint of the rise gives the anatomical dead space, about 150 mL. The slope of the plateau and the later upswing reflect uneven ventilation and closing volume.

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: single-breath nitrogen test?

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.