Genetic and metabolic disorders: McArdle disease: practice question

A Pediatrics 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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Pediatrics · Genetic and metabolic disorders: McArdle disease

Sample item

An adolescent has painful muscle cramps and myoglobinuria on strenuous exercise, but after a brief rest can continue exercising comfortably (second wind). Which enzyme is deficient?

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

The answer and the reasoning

C. Muscle glycogen phosphorylase

McArdle disease (GSD V) is caused by muscle phosphorylase deficiency, with exercise intolerance, cramps and a characteristic second wind phenomenon. Lactate does not rise during ischaemic forearm exercise.

Option by option

  • A. Debranching enzyme

    Debranching enzyme deficiency causes Cori disease.

  • B. Acid alpha-glucosidase

    Acid alpha-glucosidase deficiency causes Pompe disease.

  • C. Muscle glycogen phosphorylase (correct)

    Correct. Muscle phosphorylase deficiency causes McArdle disease.

  • D. Glucose-6-phosphatase

    Glucose-6-phosphatase deficiency causes von Gierke disease.

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 Pediatrics question

What is the answer to this Pediatrics question?

The best answer is C, Muscle glycogen phosphorylase. McArdle disease (GSD V) is caused by muscle phosphorylase deficiency, with exercise intolerance, cramps and a characteristic second wind phenomenon.

Why is "Debranching enzyme" not the answer?

Debranching enzyme deficiency causes Cori disease.

Why is "Acid alpha-glucosidase" not the answer?

Acid alpha-glucosidase deficiency causes Pompe disease.

Why is "Glucose-6-phosphatase" not the answer?

Glucose-6-phosphatase deficiency causes von Gierke disease.

What concept does this question test?

It tests Genetic and metabolic disorders: McArdle disease in Pediatrics. Lactate does not rise during ischaemic forearm exercise.

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 Genetic and metabolic disorders: McArdle disease?

Memorise milestones and the vaccine schedule as tables and revise them often. Then use the flashcards and the other Pediatrics 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.