Complications and emergencies: methylene blue and enzyme deficiency: practice question

A Anaesthesia 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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Anaesthesia · Complications and emergencies: methylene blue and enzyme deficiency

Sample item

Methylene blue is relatively contraindicated for treating methaemoglobinaemia in patients deficient in which red cell enzyme, because it may fail to work and can precipitate haemolysis?

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

The answer and the reasoning

A. Glucose-6-phosphate dehydrogenase

Methylene blue is converted to its active form, leucomethylene blue, by an NADPH-dependent reductase. G6PD deficiency leaves too little NADPH, so the drug is ineffective and the oxidant stress can cause haemolysis. Ascorbic acid or exchange transfusion are alternatives.

Option by option

  • A. Glucose-6-phosphate dehydrogenase (correct)

    Correct. Without G6PD there is not enough NADPH to activate methylene blue, and haemolysis can follow.

  • B. Plasma cholinesterase (pseudocholinesterase)

    Pseudocholinesterase deficiency causes prolonged suxamethonium and mivacurium block and is unrelated to methylene blue.

  • C. Methaemoglobin reductase of the cytochrome b5 pathway

    The cytochrome b5 pathway is the normal slow route for methaemoglobin reduction, and deficiency is the cause of hereditary methaemoglobinaemia that methylene blue does treat.

  • D. Erythrocyte pyruvate kinase enzyme

    Pyruvate kinase deficiency is a different cause of haemolytic anaemia and is not the reason for avoiding methylene blue.

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 Anaesthesia question

What is the answer to this Anaesthesia question?

The best answer is A, Glucose-6-phosphate dehydrogenase. Methylene blue is converted to its active form, leucomethylene blue, by an NADPH-dependent reductase.

Why is "Plasma cholinesterase (pseudocholinesterase)" not the answer?

Pseudocholinesterase deficiency causes prolonged suxamethonium and mivacurium block and is unrelated to methylene blue.

Why is "Methaemoglobin reductase of the cytochrome b5 pathway" not the answer?

The cytochrome b5 pathway is the normal slow route for methaemoglobin reduction, and deficiency is the cause of hereditary methaemoglobinaemia that methylene blue does treat.

Why is "Erythrocyte pyruvate kinase enzyme" not the answer?

Pyruvate kinase deficiency is a different cause of haemolytic anaemia and is not the reason for avoiding methylene blue.

What concept does this question test?

It tests Complications and emergencies: methylene blue and enzyme deficiency in Anaesthesia. G6PD deficiency leaves too little NADPH, so the drug is ineffective and the oxidant stress can cause haemolysis. Ascorbic acid or exchange transfusion are alternatives.

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 Complications and emergencies: methylene blue and enzyme deficiency?

Make a table of each agent, its key advantage and its main side effect. Then use the flashcards and the other Anaesthesia 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.