Blood and immunity: 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 · Blood and immunity: carbon monoxide poisoning

Sample item

A family is brought in from a closed room with a coal stove, with headache and drowsiness. Pulse oximetry reads 98 percent but the patient is clearly unwell. Which statement best explains the findings?

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

The answer and the reasoning

B. Carboxyhaemoglobin is read as oxyhaemoglobin, while oxygen content is low and the curve is shifted left

Carbon monoxide binds haemoglobin with about 200 times the affinity of oxygen. Standard two-wavelength oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so saturation looks normal. Oxygen content is low and the curve shifts left, so release is impaired. Treat with 100 percent oxygen.

Option by option

  • A. Dissolved oxygen is raised and compensates for the saturation

    Dissolved oxygen is normal and too small to compensate.

  • B. Carboxyhaemoglobin is read as oxyhaemoglobin, while oxygen content is low and the curve is shifted left (correct)

    Correct. Carboxyhaemoglobin falsely reads as saturated, oxygen content is low and release is impaired.

  • C. Haemoglobin concentration is falling because of haemolysis

    Haemoglobin concentration is normal; the problem is its occupancy by carbon monoxide.

  • D. Arterial PO2 is severely reduced although saturation looks normal

    PaO2 is normal in carbon monoxide poisoning because dissolved oxygen is unaffected.

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 B, Carboxyhaemoglobin is read as oxyhaemoglobin, while oxygen content is low and the curve is shifted left. Carbon monoxide binds haemoglobin with about 200 times the affinity of oxygen.

Why is "Dissolved oxygen is raised and compensates for the saturation" not the answer?

Dissolved oxygen is normal and too small to compensate.

Why is "Haemoglobin concentration is falling because of haemolysis" not the answer?

Haemoglobin concentration is normal; the problem is its occupancy by carbon monoxide.

Why is "Arterial PO2 is severely reduced although saturation looks normal" not the answer?

PaO2 is normal in carbon monoxide poisoning because dissolved oxygen is unaffected.

What concept does this question test?

It tests Blood and immunity: carbon monoxide poisoning in Physiology. Standard two-wavelength oximeters cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so saturation looks normal. Oxygen content is low and the curve shifts left, so release is impaired. Treat with 100 percent 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 Blood and immunity: 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.