Lipid metabolism: phosphatidylserine externalisation in apoptosis: 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 · Lipid metabolism: phosphatidylserine externalisation in apoptosis

Sample item

Annexin V staining is used to detect early apoptotic cells. Which membrane phospholipid, normally confined to the inner leaflet, is exposed on the cell surface and bound by annexin V?

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

The answer and the reasoning

D. Phosphatidylserine

In healthy cells phosphatidylserine is kept on the cytoplasmic leaflet by flippases. During apoptosis it is exposed on the outer surface, acting as an eat-me signal for phagocytes and binding annexin V in the presence of calcium. Phosphatidylcholine and sphingomyelin are mainly on the outer leaflet in healthy cells.

Option by option

  • A. Sphingomyelin

    Sphingomyelin is predominantly on the outer leaflet of healthy cells.

  • B. Phosphatidylinositol

    Phosphatidylinositol is a minor inner leaflet lipid used in signalling, and annexin V does not bind it.

  • C. Phosphatidylcholine

    Phosphatidylcholine is mainly in the outer leaflet already and is not the annexin V ligand.

  • D. Phosphatidylserine (correct)

    Correct. Externalised phosphatidylserine is the target of annexin V.

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 D, Phosphatidylserine. In healthy cells phosphatidylserine is kept on the cytoplasmic leaflet by flippases.

Why is "Sphingomyelin" not the answer?

Sphingomyelin is predominantly on the outer leaflet of healthy cells.

Why is "Phosphatidylinositol" not the answer?

Phosphatidylinositol is a minor inner leaflet lipid used in signalling, and annexin V does not bind it.

Why is "Phosphatidylcholine" not the answer?

Phosphatidylcholine is mainly in the outer leaflet already and is not the annexin V ligand.

What concept does this question test?

It tests Lipid metabolism: phosphatidylserine externalisation in apoptosis in Biochemistry. During apoptosis it is exposed on the outer surface, acting as an eat-me signal for phagocytes and binding annexin V in the presence of calcium. Phosphatidylcholine and sphingomyelin are mainly on the outer leaflet in healthy cells.

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 Lipid metabolism: phosphatidylserine externalisation in apoptosis?

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.