Glycoproteins: blood group biochemistry: 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 · Glycoproteins: blood group biochemistry

Sample item

A woman whose red cells type as group O has serum that agglutinates group O red cells from other donors. Her daughter, fathered by a group A man, is group AB. Which defect best explains the mother's phenotype?

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

The answer and the reasoning

B. Inability to add fucose to the precursor chain, so H antigen is absent and A and B antigens cannot form

ABO antigens are built on the H antigen, formed when fucosyltransferase (FUT1) adds fucose to the precursor oligosaccharide. In the Bombay phenotype (hh) no H antigen is made, so red cells type as O even when ABO genes such as B are inherited, and the serum contains anti-H that agglutinates normal group O cells. Such a woman can pass on a B gene, explaining an AB child.

Option by option

  • A. Mutation in the RHD gene

    RHD mutations alter Rh D status and do not affect ABO antigens.

  • B. Inability to add fucose to the precursor chain, so H antigen is absent and A and B antigens cannot form (correct)

    Correct. Lack of H antigen blocks A and B expression, and the serum makes anti-H.

  • C. Absence of N-acetylgalactosaminyltransferase, so A antigen cannot be formed

    Lack of the A transferase gives group B or O, and the serum would not react with ordinary group O cells.

  • D. Absence of galactosyltransferase only, so B antigen cannot be formed

    Lack of the B transferase gives group A or O, and the serum would not react with ordinary group O cells.

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, Inability to add fucose to the precursor chain, so H antigen is absent and A and B antigens cannot form. ABO antigens are built on the H antigen, formed when fucosyltransferase (FUT1) adds fucose to the precursor oligosaccharide.

Why is "Mutation in the RHD gene" not the answer?

RHD mutations alter Rh D status and do not affect ABO antigens.

Why is "Absence of N-acetylgalactosaminyltransferase, so A antigen cannot be…" not the answer?

Lack of the A transferase gives group B or O, and the serum would not react with ordinary group O cells.

Why is "Absence of galactosyltransferase only, so B antigen cannot be formed" not the answer?

Lack of the B transferase gives group A or O, and the serum would not react with ordinary group O cells.

What concept does this question test?

It tests Glycoproteins: blood group biochemistry in Biochemistry. In the Bombay phenotype (hh) no H antigen is made, so red cells type as O even when ABO genes such as B are inherited, and the serum contains anti-H that agglutinates normal group O cells. Such a woman can pass on a B gene, explaining an AB child.

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 Glycoproteins: blood group biochemistry?

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.