Respiratory system: carbon monoxide poisoning: 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: carbon monoxide poisoning

Sample item

A 28-year-old man is found drowsy and confused beside a burning charcoal heater in a closed room in winter. Pulse oximetry reads 98% and arterial PaO2 is 95 mmHg. Which statement explains the discrepancy with his clinical state?

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

The answer and the reasoning

C. Carboxyhaemoglobin is read as oxyhaemoglobin, and oxygen content falls despite normal PaO2

Carbon monoxide binds haemoglobin with an affinity about 200 times that of oxygen. This reduces oxyhaemoglobin and total oxygen content and shifts the dissociation curve to the left, so the remaining oxygen is released poorly. PaO2 reflects only dissolved oxygen and remains normal, and two-wavelength pulse oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin.

Option by option

  • A. Normal PaO2 and saturation show that oxygen delivery to the tissues is adequate

    Oxygen content depends on haemoglobin available to carry oxygen, not on PaO2 alone.

  • B. Carbon monoxide forms methaemoglobin, which reads falsely high on oximetry

    Methaemoglobinaemia tends to drive the oximeter reading toward about 85% and is not the effect of carbon monoxide.

  • C. Carboxyhaemoglobin is read as oxyhaemoglobin, and oxygen content falls despite normal PaO2 (correct)

    Co-oximetry or an arterial carboxyhaemoglobin level is needed to reveal the true state.

  • D. Carbon monoxide shifts the curve rightward, so tissues receive more oxygen than normal

    The curve shifts to the left, so tissues receive less oxygen at a given PO2.

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, Carboxyhaemoglobin is read as oxyhaemoglobin, and oxygen content falls despite normal PaO2. Carbon monoxide binds haemoglobin with an affinity about 200 times that of oxygen.

Why is "Normal PaO2 and saturation show that oxygen delivery to the tissues i…" not the answer?

Oxygen content depends on haemoglobin available to carry oxygen, not on PaO2 alone.

Why is "Carbon monoxide forms methaemoglobin, which reads falsely high on oxi…" not the answer?

Methaemoglobinaemia tends to drive the oximeter reading toward about 85% and is not the effect of carbon monoxide.

Why is "Carbon monoxide shifts the curve rightward, so tissues receive more o…" not the answer?

The curve shifts to the left, so tissues receive less oxygen at a given PO2.

What concept does this question test?

It tests Respiratory system: carbon monoxide poisoning in Physiology. This reduces oxyhaemoglobin and total oxygen content and shifts the dissociation curve to the left, so the remaining oxygen is released poorly. PaO2 reflects only dissolved oxygen and remains normal, and two-wavelength pulse oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin.

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: carbon monoxide poisoning?

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.