Asphyxial deaths: carbon monoxide and the dissociation curve: 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 · Asphyxial deaths: carbon monoxide and the dissociation curve

Sample item

Why is carbon monoxide poisoning more dangerous than an equal fall in haemoglobin from anaemia, even though the oxygen content may be reduced to the same extent?

AThe left shift of the curve holds oxygen on the remaining haemoglobinBIt haemolyses red cells and causes a secondary fall in haemoglobinCIt oxidises haem iron to the ferric formDThe right shift of the curve dumps oxygen before it reaches tissues

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

The answer and the reasoning

A. The left shift of the curve holds oxygen on the remaining haemoglobin

Carbon monoxide binds haemoglobin with an affinity about 200 times that of oxygen. It also shifts the oxygen dissociation curve to the left, so the remaining oxyhaemoglobin releases oxygen poorly to the tissues. Anaemia lowers carrying capacity but does not impair release.

Option by option

A. The left shift of the curve holds oxygen on the remaining haemoglobin (correct)
Correct. A left-shifted curve limits the release of oxygen in addition to the lost carrying capacity.
B. It haemolyses red cells and causes a secondary fall in haemoglobin
Haemolysis is not a feature of carbon monoxide poisoning.
C. It oxidises haem iron to the ferric form
Ferric iron is formed in methaemoglobinaemia, not in carboxyhaemoglobin formation.
D. The right shift of the curve dumps oxygen before it reaches tissues
Carbon monoxide causes a left shift, not a right shift.

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 A, The left shift of the curve holds oxygen on the remaining haemoglobin. Carbon monoxide binds haemoglobin with an affinity about 200 times that of oxygen.

Why is "It haemolyses red cells and causes a secondary fall in haemoglobin" not the answer?

Haemolysis is not a feature of carbon monoxide poisoning.

Why is "It oxidises haem iron to the ferric form" not the answer?

Ferric iron is formed in methaemoglobinaemia, not in carboxyhaemoglobin formation.

Why is "The right shift of the curve dumps oxygen before it reaches tissues" not the answer?

Carbon monoxide causes a left shift, not a right shift.

What concept does this question test?

It tests Asphyxial deaths: carbon monoxide and the dissociation curve in Forensic Medicine. It also shifts the oxygen dissociation curve to the left, so the remaining oxyhaemoglobin releases oxygen poorly to the tissues. Anaemia lowers carrying capacity but does not impair release.

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 Asphyxial deaths: carbon monoxide and the dissociation curve?

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.