Immunology: phagocyte disorders: practice question

A Microbiology 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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Microbiology · Immunology: phagocyte disorders

Sample item

A 5-year-old boy has recurrent abscesses of the skin, lymph nodes and liver caused by Staphylococcus aureus and Aspergillus. The nitroblue tetrazolium test remains colourless. Which defect is present?

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

The answer and the reasoning

A. Deficiency of NADPH oxidase, preventing the respiratory burst

Chronic granulomatous disease results from NADPH oxidase defects (most commonly X-linked gp91phox), so phagocytes cannot generate superoxide. Catalase-positive organisms such as Staphylococcus aureus, Aspergillus, Nocardia and Serratia are especially problematic because they destroy their own hydrogen peroxide. The NBT and dihydrorhodamine tests are abnormal.

Option by option

  • A. Deficiency of NADPH oxidase, preventing the respiratory burst (correct)

    Correct. Failure of the oxidative burst leaves the NBT test unchanged.

  • B. Deficiency of the C5-C9 complement components

    Terminal complement deficiency predisposes to Neisseria infections.

  • C. Deficiency of myeloperoxidase only

    Myeloperoxidase deficiency usually causes few infections, other than Candida in diabetics.

  • D. Defective LFA-1 (CD18) adhesion

    LAD-1 causes delayed umbilical cord separation and leucocytosis without pus.

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 Microbiology question

What is the answer to this Microbiology question?

The best answer is A, Deficiency of NADPH oxidase, preventing the respiratory burst. Chronic granulomatous disease results from NADPH oxidase defects (most commonly X-linked gp91phox), so phagocytes cannot generate superoxide.

Why is "Deficiency of the C5-C9 complement components" not the answer?

Terminal complement deficiency predisposes to Neisseria infections.

Why is "Deficiency of myeloperoxidase only" not the answer?

Myeloperoxidase deficiency usually causes few infections, other than Candida in diabetics.

Why is "Defective LFA-1 (CD18) adhesion" not the answer?

LAD-1 causes delayed umbilical cord separation and leucocytosis without pus.

What concept does this question test?

It tests Immunology: phagocyte disorders in Microbiology. Catalase-positive organisms such as Staphylococcus aureus, Aspergillus, Nocardia and Serratia are especially problematic because they destroy their own hydrogen peroxide. The NBT and dihydrorhodamine tests are abnormal.

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 Immunology: phagocyte disorders?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology 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.