Toxicology: principle of the breath analyser: practice question

A Forensic Medicine 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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Forensic Medicine · Toxicology: principle of the breath analyser

Sample item

A traffic police officer uses a breath analyser to estimate a driver's blood alcohol. Which principle allows breath alcohol to be converted into a blood level?

AAlcohol is converted to carbon dioxide in the lungsBAlcohol partitions between blood and alveolar air in a constant ratioCAlcohol in saliva equals the level in the stomach contentsDAlcohol is bound to haemoglobin and released in expired air

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

The answer and the reasoning

B. Alcohol partitions between blood and alveolar air in a constant ratio

At body temperature, alcohol in pulmonary capillary blood equilibrates with alveolar air in a fixed proportion, in accordance with Henry's law. The concentration in end-expired breath therefore reflects the blood level. Mouth alcohol and a poor exhalation can give erroneous readings.

Option by option

A. Alcohol is converted to carbon dioxide in the lungs
Alcohol is metabolised in the liver, not converted in the lungs.
B. Alcohol partitions between blood and alveolar air in a constant ratio (correct)
Correct. The constant blood-to-breath ratio makes conversion possible.
C. Alcohol in saliva equals the level in the stomach contents
Saliva and stomach contents are not the basis of the breath test.
D. Alcohol is bound to haemoglobin and released in expired air
Alcohol does not bind to haemoglobin.

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 Forensic Medicine question

What is the answer to this Forensic Medicine question?

The best answer is B, Alcohol partitions between blood and alveolar air in a constant ratio. At body temperature, alcohol in pulmonary capillary blood equilibrates with alveolar air in a fixed proportion, in accordance with Henry's law.

Why is "Alcohol is converted to carbon dioxide in the lungs" not the answer?

Alcohol is metabolised in the liver, not converted in the lungs.

Why is "Alcohol in saliva equals the level in the stomach contents" not the answer?

Saliva and stomach contents are not the basis of the breath test.

Why is "Alcohol is bound to haemoglobin and released in expired air" not the answer?

Alcohol does not bind to haemoglobin.

What concept does this question test?

It tests Toxicology: principle of the breath analyser in Forensic Medicine. The concentration in end-expired breath therefore reflects the blood level. Mouth alcohol and a poor exhalation can give erroneous readings.

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 Toxicology: principle of the breath analyser?

Learn postmortem changes with their timelines, and toxicology as poison, sign, test and antidote. Then use the flashcards and the other Forensic Medicine 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.