Glycogen metabolism: McArdle disease: practice question

A Biochemistry 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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Biochemistry · Glycogen metabolism: McArdle disease

Sample item

A teenager has painful cramps with brief intense exercise, myoglobinuria, and a normal rise in lactate is absent after exercise. 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 is caused by muscle glycogen phosphorylase deficiency. Muscle cannot mobilise glycogen during exertion, but liver glucose release remains intact.

Option by option

  • A. Lysosomal alpha-glucosidase

    Alpha-glucosidase deficiency causes Pompe disease with cardiomyopathy.

  • B. Glucose-6-phosphatase

    Glucose-6-phosphatase deficiency causes severe fasting hypoglycaemia in von Gierke disease.

  • C. Muscle glycogen phosphorylase (correct)

    Correct. Myophosphorylase deficiency causes exercise intolerance and a flat lactate response.

  • D. Branching enzyme

    Branching-enzyme deficiency causes Andersen 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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Muscle glycogen phosphorylase. McArdle disease is caused by muscle glycogen phosphorylase deficiency.

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

Alpha-glucosidase deficiency causes Pompe disease with cardiomyopathy.

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

Glucose-6-phosphatase deficiency causes severe fasting hypoglycaemia in von Gierke disease.

Why is "Branching enzyme" not the answer?

Branching-enzyme deficiency causes Andersen disease.

What concept does this question test?

It tests Glycogen metabolism: McArdle disease in Biochemistry. Muscle cannot mobilise glycogen during exertion, but liver glucose release remains intact.

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 Glycogen metabolism: McArdle disease?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.