Carbohydrate metabolism: glycogen storage disease type Ib: 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 · Carbohydrate metabolism: glycogen storage disease type Ib

Sample item

A 2-year-old with hepatomegaly, fasting hypoglycaemia, lactic acidosis and hyperuricaemia also has recurrent bacterial infections, oral ulcers and neutropenia. Glucose-6-phosphatase activity is normal when the liver sample is disrupted. Which protein is defective?

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

The answer and the reasoning

B. Glucose-6-phosphate translocase

In type Ib glycogen storage disease the transporter that carries glucose-6-phosphate into the endoplasmic reticulum is defective. The enzyme itself is normal once the membranes are disrupted. The same defect impairs neutrophil function, causing neutropenia and infections.

Option by option

  • A. Phosphorylase kinase

    Phosphorylase kinase deficiency causes mild hepatomegaly with growth delay and does not cause lactic acidosis or neutropenia.

  • B. Glucose-6-phosphate translocase (correct)

    Correct. Defective transport of glucose-6-phosphate into the endoplasmic reticulum explains the von Gierke picture plus neutropenia.

  • C. Catalytic subunit of glucose-6-phosphatase

    A defect in the catalytic subunit is type Ia, which has the same metabolic features but normal neutrophil counts, and activity is low even in disrupted tissue.

  • D. Glycogen debranching enzyme

    Debranching enzyme deficiency causes type III disease, with milder hypoglycaemia, normal lactate and abnormal limit dextrin.

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, Glucose-6-phosphate translocase. In type Ib glycogen storage disease the transporter that carries glucose-6-phosphate into the endoplasmic reticulum is defective.

Why is "Phosphorylase kinase" not the answer?

Phosphorylase kinase deficiency causes mild hepatomegaly with growth delay and does not cause lactic acidosis or neutropenia.

Why is "Catalytic subunit of glucose-6-phosphatase" not the answer?

A defect in the catalytic subunit is type Ia, which has the same metabolic features but normal neutrophil counts, and activity is low even in disrupted tissue.

Why is "Glycogen debranching enzyme" not the answer?

Debranching enzyme deficiency causes type III disease, with milder hypoglycaemia, normal lactate and abnormal limit dextrin.

What concept does this question test?

It tests Carbohydrate metabolism: glycogen storage disease type Ib in Biochemistry. The enzyme itself is normal once the membranes are disrupted. The same defect impairs neutrophil function, causing neutropenia and infections.

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 Carbohydrate metabolism: glycogen storage disease type Ib?

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.