Glycogen storage disease: McArdle: 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 storage disease: McArdle

Sample item

A young adult has exercise intolerance, muscle cramps, myoglobinuria, and no rise in serum lactate after ischemic exercise testing. What deficiency is likely?

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

The answer and the reasoning

B. Muscle glycogen phosphorylase deficiency

McArdle disease is muscle glycogen phosphorylase deficiency, preventing glycogenolysis in skeletal muscle during exercise. Patients may experience a second-wind phenomenon after switching to circulating fuels.

Option by option

  • A. Hexosaminidase A deficiency

    Hexosaminidase A deficiency causes Tay-Sachs disease.

  • B. Muscle glycogen phosphorylase deficiency (correct)

    Correct. Lack of muscle glycogen mobilization explains exercise cramps and myoglobinuria.

  • C. Liver glycogen phosphorylase deficiency

    Liver phosphorylase deficiency mainly causes mild fasting hypoglycemia and hepatomegaly.

  • D. Pyruvate carboxylase deficiency

    Pyruvate carboxylase deficiency causes lactic acidosis and neurologic 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 B, Muscle glycogen phosphorylase deficiency. McArdle disease is muscle glycogen phosphorylase deficiency, preventing glycogenolysis in skeletal muscle during exercise.

Why is "Hexosaminidase A deficiency" not the answer?

Hexosaminidase A deficiency causes Tay-Sachs disease.

Why is "Liver glycogen phosphorylase deficiency" not the answer?

Liver phosphorylase deficiency mainly causes mild fasting hypoglycemia and hepatomegaly.

Why is "Pyruvate carboxylase deficiency" not the answer?

Pyruvate carboxylase deficiency causes lactic acidosis and neurologic disease.

What concept does this question test?

It tests Glycogen storage disease: McArdle in Biochemistry. Patients may experience a second-wind phenomenon after switching to circulating fuels.

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 storage disease: McArdle?

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.