Haemoglobinopathies: alpha thalassaemia: 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 · Haemoglobinopathies: alpha thalassaemia

Sample item

A stillborn hydropic fetus of a couple of Southeast Asian ancestry has massive hepatosplenomegaly and haemoglobin electrophoresis showing predominantly Hb Bart's. Which genotype is expected?

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

The answer and the reasoning

A. Deletion of all four alpha-globin genes

Each chromosome 16 carries two alpha-globin genes, so four copies are present in total. Loss of all four prevents alpha chain synthesis, and gamma chains form tetramers (Hb Bart's) that have very high oxygen affinity and cannot deliver oxygen, causing hydrops fetalis. Deletion of three genes gives Hb H disease, a milder, chronic haemolytic anaemia.

Option by option

  • A. Deletion of all four alpha-globin genes (correct)

    Complete alpha gene loss leaves only gamma tetramers and causes hydrops.

  • B. Deletion of three alpha-globin genes

    Loss of three genes causes Hb H disease, which is compatible with survival.

  • C. Deletion of one alpha-globin gene

    A single gene deletion is a silent carrier state.

  • D. Homozygous beta-globin point mutation

    A homozygous beta mutation causes beta thalassaemia major after the neonatal period, when HbA should appear.

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 A, Deletion of all four alpha-globin genes. Each chromosome 16 carries two alpha-globin genes, so four copies are present in total.

Why is "Deletion of three alpha-globin genes" not the answer?

Loss of three genes causes Hb H disease, which is compatible with survival.

Why is "Deletion of one alpha-globin gene" not the answer?

A single gene deletion is a silent carrier state.

Why is "Homozygous beta-globin point mutation" not the answer?

A homozygous beta mutation causes beta thalassaemia major after the neonatal period, when HbA should appear.

What concept does this question test?

It tests Haemoglobinopathies: alpha thalassaemia in Biochemistry. Loss of all four prevents alpha chain synthesis, and gamma chains form tetramers (Hb Bart's) that have very high oxygen affinity and cannot deliver oxygen, causing hydrops fetalis. Deletion of three genes gives Hb H disease, a milder, chronic haemolytic anaemia.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Haemoglobinopathies: alpha thalassaemia?

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.