Glycogen storage disorders: 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 disorders

Sample item

A 20-year-old man has painful cramps and fatigue within minutes of starting vigorous exercise, and dark urine afterwards. During an ischaemic forearm exercise test his venous lactate does not rise, though ammonia does. What is the likely enzyme defect?

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

The answer and the reasoning

A. Muscle glycogen phosphorylase (McArdle disease)

In McArdle disease (type V) muscle cannot break down glycogen, so anaerobic glycolysis cannot supply energy. Exercise causes cramps and sometimes myoglobinuria, and lactate does not rise on the forearm test. A 'second wind' phenomenon occurs as blood-borne fuels are mobilised.

Option by option

  • A. Muscle glycogen phosphorylase (McArdle disease) (correct)

    Correct. Absent muscle phosphorylase prevents lactate production from glycogen.

  • B. Glucose-6-phosphatase

    Glucose-6-phosphatase deficiency causes fasting hypoglycaemia and hepatomegaly (Von Gierke disease).

  • C. Lysosomal acid alpha-glucosidase

    Acid alpha-glucosidase deficiency causes Pompe disease with cardiomyopathy and hypotonia in infants.

  • D. Muscle carnitine palmitoyltransferase II

    CPT II deficiency causes myoglobinuria after prolonged exercise or fasting, but lactate rises normally because glycolysis is intact.

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 A, Muscle glycogen phosphorylase (McArdle disease). In McArdle disease (type V) muscle cannot break down glycogen, so anaerobic glycolysis cannot supply energy.

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

Glucose-6-phosphatase deficiency causes fasting hypoglycaemia and hepatomegaly (Von Gierke disease).

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

Acid alpha-glucosidase deficiency causes Pompe disease with cardiomyopathy and hypotonia in infants.

Why is "Muscle carnitine palmitoyltransferase II" not the answer?

CPT II deficiency causes myoglobinuria after prolonged exercise or fasting, but lactate rises normally because glycolysis is intact.

What concept does this question test?

It tests Glycogen storage disorders in Biochemistry. Exercise causes cramps and sometimes myoglobinuria, and lactate does not rise on the forearm test. A 'second wind' phenomenon occurs as blood-borne fuels are mobilised.

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 disorders?

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.