Enzyme inhibition: irreversible inhibition by aspirin: 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 · Enzyme inhibition: irreversible inhibition by aspirin

Sample item

A man takes aspirin 75 mg daily. A bleeding time measured three days after he stops aspirin is still prolonged, although aspirin has a plasma half-life of only about 20 minutes. Which statement best explains this?

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

The answer and the reasoning

D. Aspirin irreversibly acetylates a serine of cyclooxygenase-1, and anucleate platelets cannot make new enzyme

Aspirin is a covalent, irreversible inhibitor: it transfers an acetyl group to a serine near the active site of COX-1, blocking arachidonate access and thromboxane A2 synthesis. Platelets have no nucleus, so they cannot make fresh enzyme and the block lasts for the 7 to 10 day platelet life span. Function returns only as new platelets enter the circulation.

Option by option

  • A. Aspirin binds the cyclooxygenase active site reversibly but dissociates from it very slowly

    Slow dissociation would still allow recovery within hours or days of plasma clearance; the block here is covalent.

  • B. Aspirin damages megakaryocyte DNA so that newly formed platelets lack thromboxane synthase

    Aspirin does not damage megakaryocyte DNA, and new platelets released later have normal COX-1.

  • C. Aspirin is stored in platelet granules and is released from them slowly over several days

    Aspirin is not stored in granules; its action depends on covalent modification of cyclooxygenase in the platelet.

  • D. Aspirin irreversibly acetylates a serine of cyclooxygenase-1, and anucleate platelets cannot make new enzyme (correct)

    Covalent acetylation of serine in COX-1 cannot be reversed in an anucleate platelet, so the effect outlasts the drug.

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, Aspirin irreversibly acetylates a serine of cyclooxygenase-1, and anucleate platelets cannot make new enzyme. Aspirin is a covalent, irreversible inhibitor: it transfers an acetyl group to a serine near the active site of COX-1, blocking arachidonate access and thromboxane A2 synthesis.

Why is "Aspirin binds the cyclooxygenase active site reversibly but dissociat…" not the answer?

Slow dissociation would still allow recovery within hours or days of plasma clearance; the block here is covalent.

Why is "Aspirin damages megakaryocyte DNA so that newly formed platelets lack…" not the answer?

Aspirin does not damage megakaryocyte DNA, and new platelets released later have normal COX-1.

Why is "Aspirin is stored in platelet granules and is released from them slow…" not the answer?

Aspirin is not stored in granules; its action depends on covalent modification of cyclooxygenase in the platelet.

What concept does this question test?

It tests Enzyme inhibition: irreversible inhibition by aspirin in Biochemistry. Platelets have no nucleus, so they cannot make fresh enzyme and the block lasts for the 7 to 10 day platelet life span. Function returns only as new platelets enter the circulation.

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 Enzyme inhibition: irreversible inhibition by aspirin?

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.