Cell injury: cyanide poisoning: practice question

A Pathology 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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Pathology · Cell injury: cyanide poisoning

Sample item

A worker is found collapsed after drinking an electroplating solution. He has cherry-red skin and severe lactic acidosis, venous blood is bright red, and PaO2 is normal. Which step of cellular respiration is blocked, and why is lactate high?

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

The answer and the reasoning

B. Cytochrome c oxidase is inhibited, halting oxidative phosphorylation and forcing anaerobic glycolysis

Cyanide binds the ferric iron of cytochrome c oxidase (complex IV), so tissues cannot use oxygen even though delivery is normal. Venous blood stays oxygen rich, hence the bright red colour and narrow arteriovenous oxygen difference. Cells shift to anaerobic glycolysis, producing lactic acidosis.

Option by option

  • A. Carboxyhaemoglobin formation limits oxygen carriage while tissue oxygen utilisation stays normal

    Carbon monoxide poisoning also gives cherry-red skin but reduces oxygen content and carriage, and the setting here is an electroplating cyanide solution.

  • B. Cytochrome c oxidase is inhibited, halting oxidative phosphorylation and forcing anaerobic glycolysis (correct)

    Correct. Blocked complex IV explains normal PaO2, bright venous blood and lactic acidosis.

  • C. Methaemoglobin formation prevents oxygen binding, so tissues extract oxygen normally from plasma

    Methaemoglobinaemia produces chocolate-brown blood and cyanosis, and does not match bright red venous blood.

  • D. Mitochondrial proton gradient uncoupling dissipates energy as heat, causing hyperthermia

    Uncoupling, as with dinitrophenol, causes hyperthermia with increased oxygen consumption, not a narrowed arteriovenous oxygen difference.

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 Pathology question

What is the answer to this Pathology question?

The best answer is B, Cytochrome c oxidase is inhibited, halting oxidative phosphorylation and forcing anaerobic glycolysis. Cyanide binds the ferric iron of cytochrome c oxidase (complex IV), so tissues cannot use oxygen even though delivery is normal.

Why is "Carboxyhaemoglobin formation limits oxygen carriage while tissue oxyg…" not the answer?

Carbon monoxide poisoning also gives cherry-red skin but reduces oxygen content and carriage, and the setting here is an electroplating cyanide solution.

Why is "Methaemoglobin formation prevents oxygen binding, so tissues extract…" not the answer?

Methaemoglobinaemia produces chocolate-brown blood and cyanosis, and does not match bright red venous blood.

Why is "Mitochondrial proton gradient uncoupling dissipates energy as heat, c…" not the answer?

Uncoupling, as with dinitrophenol, causes hyperthermia with increased oxygen consumption, not a narrowed arteriovenous oxygen difference.

What concept does this question test?

It tests Cell injury: cyanide poisoning in Pathology. Venous blood stays oxygen rich, hence the bright red colour and narrow arteriovenous oxygen difference. Cells shift to anaerobic glycolysis, producing lactic acidosis.

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 Cell injury: cyanide poisoning?

Learn each disease as a bundle: cause, key histology, markers, and the clinical hallmark. Then use the flashcards and the other Pathology 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.