Glycolysis: muscle phosphofructokinase deficiency: 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 · Glycolysis: muscle phosphofructokinase deficiency

Sample item

A 25-year-old man has exercise intolerance with cramps that worsen if he takes a glucose-rich snack before exertion. He also has mild chronic haemolysis with raised reticulocytes. Muscle biopsy shows glycogen accumulation, and forearm ischaemic exercise fails to raise lactate. Which enzyme is deficient?

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

The answer and the reasoning

C. Muscle phosphofructokinase-1

In Tarui disease (type VII glycogenosis) muscle phosphofructokinase-1 is absent and red cells are partially affected. Glucose loading lowers free fatty acids, which are the alternative fuel, and so worsens symptoms, unlike McArdle disease where glucose gives a second wind. Haemolysis arises because red cells depend on glycolysis.

Option by option

  • A. Muscle glycogen phosphorylase

    Muscle phosphorylase deficiency also gives no lactate rise but no haemolysis, and glucose improves exercise tolerance.

  • B. Lysosomal acid alpha-glucosidase

    Acid maltase deficiency causes a lysosomal myopathy without these exertional features or haemolysis.

  • C. Muscle phosphofructokinase-1 (correct)

    Blocked glycolysis at PFK-1 explains glycogen build-up, no lactate rise, haemolysis and worsening with glucose.

  • D. Carnitine palmitoyltransferase II

    CPT II deficiency causes myoglobinuria after prolonged exercise with a normal lactate rise.

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 phosphofructokinase-1. In Tarui disease (type VII glycogenosis) muscle phosphofructokinase-1 is absent and red cells are partially affected.

Why is "Muscle glycogen phosphorylase" not the answer?

Muscle phosphorylase deficiency also gives no lactate rise but no haemolysis, and glucose improves exercise tolerance.

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

Acid maltase deficiency causes a lysosomal myopathy without these exertional features or haemolysis.

Why is "Carnitine palmitoyltransferase II" not the answer?

CPT II deficiency causes myoglobinuria after prolonged exercise with a normal lactate rise.

What concept does this question test?

It tests Glycolysis: muscle phosphofructokinase deficiency in Biochemistry. Glucose loading lowers free fatty acids, which are the alternative fuel, and so worsens symptoms, unlike McArdle disease where glucose gives a second wind. Haemolysis arises because red cells depend on glycolysis.

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 Glycolysis: muscle phosphofructokinase deficiency?

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.