Environmental health: carbon monoxide: practice question

A PSM 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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PSM · Environmental health: carbon monoxide

Sample item

A family sleeps in a closed room with a burning coal stove during winter, and two members are found drowsy with headache and vomiting. By which mechanism does the responsible gas cause tissue hypoxia?

AIt binds haemoglobin with an affinity about 200 times that of oxygen, forming carboxyhaemoglobinBIt oxidises haemoglobin iron to the ferric stateCIt inhibits cytochrome oxidase in mitochondriaDIt causes bronchospasm and pulmonary oedema

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

The answer and the reasoning

A. It binds haemoglobin with an affinity about 200 times that of oxygen, forming carboxyhaemoglobin

Incomplete combustion of coal, wood or charcoal produces carbon monoxide. It binds haemoglobin with about 200 to 250 times the affinity of oxygen and shifts the oxygen dissociation curve to the left, so less oxygen reaches tissues. Treatment is 100% oxygen, and prevention is adequate ventilation of heating devices.

Option by option

A. It binds haemoglobin with an affinity about 200 times that of oxygen, forming carboxyhaemoglobin (correct)
Correct. Carboxyhaemoglobin formation lowers oxygen delivery.
B. It oxidises haemoglobin iron to the ferric state
Oxidation of iron to the ferric state forms methaemoglobin, as with nitrites.
C. It inhibits cytochrome oxidase in mitochondria
Cytochrome oxidase inhibition is the mechanism of cyanide poisoning.
D. It causes bronchospasm and pulmonary oedema
Bronchospasm and pulmonary oedema occur with irritant gases such as sulphur dioxide and chlorine.

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 PSM question

What is the answer to this PSM question?

The best answer is A, It binds haemoglobin with an affinity about 200 times that of oxygen, forming carboxyhaemoglobin. Incomplete combustion of coal, wood or charcoal produces carbon monoxide.

Why is "It oxidises haemoglobin iron to the ferric state" not the answer?

Oxidation of iron to the ferric state forms methaemoglobin, as with nitrites.

Why is "It inhibits cytochrome oxidase in mitochondria" not the answer?

Cytochrome oxidase inhibition is the mechanism of cyanide poisoning.

Why is "It causes bronchospasm and pulmonary oedema" not the answer?

Bronchospasm and pulmonary oedema occur with irritant gases such as sulphur dioxide and chlorine.

What concept does this question test?

It tests Environmental health: carbon monoxide in PSM. It binds haemoglobin with about 200 to 250 times the affinity of oxygen and shifts the oxygen dissociation curve to the left, so less oxygen reaches tissues. Treatment is 100% oxygen, and prevention is adequate ventilation of heating devices.

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 Environmental health: carbon monoxide?

Practise biostatistics with small calculations until sensitivity, predictive values and risk measures are automatic. Then use the flashcards and the other PSM 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.