Nucleotide drugs: cytarabine: 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 · Nucleotide drugs: cytarabine

Sample item

A 35-year-old with acute myeloid leukaemia receives cytarabine in induction chemotherapy. Which statement best describes its action?

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

The answer and the reasoning

B. It is phosphorylated by deoxycytidine kinase to ara-CTP, which competes with dCTP and inhibits DNA polymerase in S phase

Cytarabine is a cytidine analogue with arabinose in place of ribose. It enters cells, is phosphorylated by deoxycytidine kinase to ara-CTP, which is incorporated into DNA and inhibits DNA polymerase, so it is S-phase specific. Cerebellar toxicity and myelosuppression are important adverse effects.

Option by option

  • A. It inhibits IMP dehydrogenase and depletes guanine nucleotides

    IMP dehydrogenase inhibition is the action of mycophenolate.

  • B. It is phosphorylated by deoxycytidine kinase to ara-CTP, which competes with dCTP and inhibits DNA polymerase in S phase (correct)

    Ara-CTP competes with dCTP and blocks DNA chain elongation in S-phase cells.

  • C. It is converted to FdUMP and inhibits thymidylate synthase

    FdUMP formation and thymidylate synthase inhibition describes fluorouracil.

  • D. It inhibits dihydrofolate reductase and depletes tetrahydrofolate

    Dihydrofolate reductase inhibition is the action of methotrexate.

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, It is phosphorylated by deoxycytidine kinase to ara-CTP, which competes with dCTP and inhibits DNA polymerase in S phase. Cytarabine is a cytidine analogue with arabinose in place of ribose.

Why is "It inhibits IMP dehydrogenase and depletes guanine nucleotides" not the answer?

IMP dehydrogenase inhibition is the action of mycophenolate.

Why is "It is converted to FdUMP and inhibits thymidylate synthase" not the answer?

FdUMP formation and thymidylate synthase inhibition describes fluorouracil.

Why is "It inhibits dihydrofolate reductase and depletes tetrahydrofolate" not the answer?

Dihydrofolate reductase inhibition is the action of methotrexate.

What concept does this question test?

It tests Nucleotide drugs: cytarabine in Biochemistry. It enters cells, is phosphorylated by deoxycytidine kinase to ara-CTP, which is incorporated into DNA and inhibits DNA polymerase, so it is S-phase specific. Cerebellar toxicity and myelosuppression are important adverse effects.

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 Nucleotide drugs: cytarabine?

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.