Clinical biochemistry: pulse oximetry in carbon monoxide poisoning: practice question

A Biochemistry 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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Biochemistry · Clinical biochemistry: pulse oximetry in carbon monoxide poisoning

Sample item

Four members of a family are drowsy with headache after sleeping in a closed room with a coal heater. A pulse oximeter reads 98% in each. Why is the reading misleadingly normal?

APoor finger perfusion produces an artefactually high saturation valueBCarboxyhaemoglobin is read as oxyhaemoglobin by two-wavelength oximetersCFetal haemoglobin has high oxygen affinity and falsely inflates the readingDCyanide inhibition of complex IV raises venous saturation and falsifies the reading

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

The answer and the reasoning

B. Carboxyhaemoglobin is read as oxyhaemoglobin by two-wavelength oximeters

Carboxyhaemoglobin absorbs light at wavelengths similar to oxyhaemoglobin, so conventional pulse oximeters overestimate oxygen saturation. Diagnosis needs co-oximetry on an arterial or venous sample, and treatment is 100% oxygen, with hyperbaric oxygen in selected cases.

Option by option

A. Poor finger perfusion produces an artefactually high saturation value
Poor perfusion tends to cause unreliable or low readings, not a false normal.
B. Carboxyhaemoglobin is read as oxyhaemoglobin by two-wavelength oximeters (correct)
Correct. Carboxyhaemoglobin is misread as oxygenated haemoglobin.
C. Fetal haemoglobin has high oxygen affinity and falsely inflates the reading
Fetal haemoglobin is not relevant in these family members.
D. Cyanide inhibition of complex IV raises venous saturation and falsifies the reading
Cyanide is not the toxin from a coal heater and does not alter the oximeter's measurement.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Carboxyhaemoglobin is read as oxyhaemoglobin by two-wavelength oximeters. Carboxyhaemoglobin absorbs light at wavelengths similar to oxyhaemoglobin, so conventional pulse oximeters overestimate oxygen saturation.

Why is "Poor finger perfusion produces an artefactually high saturation value" not the answer?

Poor perfusion tends to cause unreliable or low readings, not a false normal.

Why is "Fetal haemoglobin has high oxygen affinity and falsely inflates the r…" not the answer?

Fetal haemoglobin is not relevant in these family members.

Why is "Cyanide inhibition of complex IV raises venous saturation and falsifi…" not the answer?

Cyanide is not the toxin from a coal heater and does not alter the oximeter's measurement.

What concept does this question test?

It tests Clinical biochemistry: pulse oximetry in carbon monoxide poisoning in Biochemistry. Diagnosis needs co-oximetry on an arterial or venous sample, and treatment is 100% oxygen, with hyperbaric oxygen in selected cases.

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 Clinical biochemistry: pulse oximetry in carbon monoxide poisoning?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.