Lipid metabolism: 5-lipoxygenase and leukotrienes: 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: 5-lipoxygenase and leukotrienes

Sample item

A drug used in asthma reduces synthesis of the leukotrienes LTC4, LTD4 and LTE4 by inhibiting an enzyme of arachidonic acid metabolism. Which enzyme does it inhibit?

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

The answer and the reasoning

D. 5-Lipoxygenase

Arachidonic acid is converted by 5-lipoxygenase to LTA4, the precursor of LTB4 and the cysteinyl leukotrienes LTC4, LTD4 and LTE4. These cause bronchoconstriction and mucus secretion and were once called the slow-reacting substance of anaphylaxis. Zileuton inhibits 5-lipoxygenase, while montelukast blocks the cysteinyl leukotriene receptor.

Option by option

  • A. Thromboxane synthase

    Thromboxane synthase makes thromboxane A2 from the prostaglandin endoperoxide.

  • B. Phospholipase A2

    Phospholipase A2 releases arachidonic acid for both pathways, so inhibiting it would not be selective for leukotrienes.

  • C. Cyclooxygenase-2

    Cyclooxygenase-2 makes prostaglandins, not leukotrienes.

  • D. 5-Lipoxygenase (correct)

    Correct. 5-Lipoxygenase starts the leukotriene pathway from arachidonic acid.

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, 5-Lipoxygenase. Arachidonic acid is converted by 5-lipoxygenase to LTA4, the precursor of LTB4 and the cysteinyl leukotrienes LTC4, LTD4 and LTE4.

Why is "Thromboxane synthase" not the answer?

Thromboxane synthase makes thromboxane A2 from the prostaglandin endoperoxide.

Why is "Phospholipase A2" not the answer?

Phospholipase A2 releases arachidonic acid for both pathways, so inhibiting it would not be selective for leukotrienes.

Why is "Cyclooxygenase-2" not the answer?

Cyclooxygenase-2 makes prostaglandins, not leukotrienes.

What concept does this question test?

It tests Lipid metabolism: 5-lipoxygenase and leukotrienes in Biochemistry. These cause bronchoconstriction and mucus secretion and were once called the slow-reacting substance of anaphylaxis. Zileuton inhibits 5-lipoxygenase, while montelukast blocks the cysteinyl leukotriene receptor.

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: 5-lipoxygenase and leukotrienes?

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.