Glycogen storage disorders: glycogen synthase 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.

Try it

Biochemistry · Glycogen storage disorders: glycogen synthase deficiency

Sample item

A 3-year-old child has early-morning drowsiness and seizures with hypoglycaemia and ketosis after an overnight fast. After a carbohydrate meal, blood glucose rises excessively and lactate is raised, and liver size and liver biopsy glycogen content are normal or reduced. Which enzyme is deficient?

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

The answer and the reasoning

B. Liver glycogen synthase

Glycogen storage disease type 0 is due to deficiency of hepatic glycogen synthase. Little glycogen can be stored after meals, so excess glucose spills into glycolysis and lactate with postprandial hyperglycaemia, while fasting hypoglycaemia occurs early because there is no glycogen reserve. Unlike other glycogen storage diseases the liver is not enlarged; frequent protein-rich feeds and uncooked cornstarch help.

Option by option

  • A. Liver glycogen phosphorylase

    Liver phosphorylase deficiency causes hepatomegaly with excess glycogen and mild hypoglycaemia.

  • B. Liver glycogen synthase (correct)

    Correct. Absent glycogen synthase leaves no storage reserve, with fasting hypoglycaemia and postprandial hyperglycaemia and lactate.

  • C. Glycogen debranching enzyme

    Debranching enzyme deficiency produces hepatomegaly with abundant abnormal glycogen of short outer chains.

  • D. Glucose-6-phosphatase

    Glucose-6-phosphatase deficiency causes hepatomegaly, lactic acidosis and hyperuricaemia from glycogen overload.

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.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Liver glycogen synthase. Glycogen storage disease type 0 is due to deficiency of hepatic glycogen synthase.

Why is "Liver glycogen phosphorylase" not the answer?

Liver phosphorylase deficiency causes hepatomegaly with excess glycogen and mild hypoglycaemia.

Why is "Glycogen debranching enzyme" not the answer?

Debranching enzyme deficiency produces hepatomegaly with abundant abnormal glycogen of short outer chains.

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

Glucose-6-phosphatase deficiency causes hepatomegaly, lactic acidosis and hyperuricaemia from glycogen overload.

What concept does this question test?

It tests Glycogen storage disorders: glycogen synthase deficiency in Biochemistry. Little glycogen can be stored after meals, so excess glucose spills into glycolysis and lactate with postprandial hyperglycaemia, while fasting hypoglycaemia occurs early because there is no glycogen reserve. Unlike other glycogen storage diseases the liver is not enlarged; frequent protein-rich feeds and uncooked cornstarch help.

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 Glycogen storage disorders: glycogen synthase 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.