Biomolecules and enzymes: congenital disorder of glycosylation 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 · Biomolecules and enzymes: congenital disorder of glycosylation type Ib

Sample item

An infant has protein-losing enteropathy, hepatic fibrosis, hypoglycaemia and a coagulopathy, but normal development and no cerebellar hypoplasia. Transferrin isoelectric focusing shows an abnormal glycoform pattern, and the child improves on oral mannose. Which enzyme is deficient?

AN-acetylglucosamine-1-phosphotransferaseBUDP-galactose 4-epimeraseCPhosphomannomutase 2DPhosphomannose isomerase

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

The answer and the reasoning

D. Phosphomannose isomerase

Phosphomannose isomerase deficiency (CDG-Ib) blocks conversion of fructose 6-phosphate to mannose 6-phosphate, so glycosylation is impaired. It is one of the few treatable glycosylation disorders because oral mannose is converted by hexokinase. Phosphomannomutase 2 deficiency (CDG-Ia) causes neurological disease instead.

Option by option

A. N-acetylglucosamine-1-phosphotransferase
N-acetylglucosamine-1-phosphotransferase deficiency causes I-cell disease, with raised plasma lysosomal enzymes.
B. UDP-galactose 4-epimerase
UDP-galactose 4-epimerase deficiency is a form of galactosaemia and does not cause an abnormal transferrin glycoform pattern with mannose responsiveness.
C. Phosphomannomutase 2
Phosphomannomutase 2 deficiency (CDG-Ia) causes cerebellar hypoplasia and developmental delay, and mannose does not help.
D. Phosphomannose isomerase (correct)
Correct. Gut and liver disease with a response to oral mannose indicates CDG-Ib.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, Phosphomannose isomerase. Phosphomannose isomerase deficiency (CDG-Ib) blocks conversion of fructose 6-phosphate to mannose 6-phosphate, so glycosylation is impaired.

Why is "N-acetylglucosamine-1-phosphotransferase" not the answer?

N-acetylglucosamine-1-phosphotransferase deficiency causes I-cell disease, with raised plasma lysosomal enzymes.

Why is "UDP-galactose 4-epimerase" not the answer?

UDP-galactose 4-epimerase deficiency is a form of galactosaemia and does not cause an abnormal transferrin glycoform pattern with mannose responsiveness.

Why is "Phosphomannomutase 2" not the answer?

Phosphomannomutase 2 deficiency (CDG-Ia) causes cerebellar hypoplasia and developmental delay, and mannose does not help.

What concept does this question test?

It tests Biomolecules and enzymes: congenital disorder of glycosylation type Ib in Biochemistry. It is one of the few treatable glycosylation disorders because oral mannose is converted by hexokinase. Phosphomannomutase 2 deficiency (CDG-Ia) causes neurological disease instead.

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 Biomolecules and enzymes: congenital disorder of glycosylation 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.