Bacteriology: Haemophilus influenzae: 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 · Bacteriology: Haemophilus influenzae

Sample item

A CSF isolate from an unimmunised child with meningitis grows poorly on sheep blood agar but forms tiny satellite colonies around a streak of Staphylococcus aureus. Which growth factors does this organism require?

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

The answer and the reasoning

D. Factor X (haemin) and factor V (NAD)

Haemophilus influenzae needs both haemin (X) and NAD (V). Sheep blood agar lacks free V factor because of NADase, but S. aureus excretes NAD, which supports growth of satellite colonies. H. ducreyi needs X only, H. parainfluenzae needs V only, and Legionella requires cysteine and iron.

Option by option

  • A. Factor V (NAD) only

    Haemophilus parainfluenzae requires V factor only, while H. influenzae also needs haemin.

  • B. Factor X (haemin) only

    Haemophilus ducreyi, the cause of chancroid, requires X factor only.

  • C. Cysteine and iron salts

    Cysteine and iron salts are the requirements of Legionella on buffered charcoal yeast extract agar.

  • D. Factor X (haemin) and factor V (NAD) (correct)

    Correct. Both X and V factors are needed, and satellitism supplies 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 Microbiology question

What is the answer to this Microbiology question?

The best answer is D, Factor X (haemin) and factor V (NAD). Haemophilus influenzae needs both haemin (X) and NAD (V).

Why is "Factor V (NAD) only" not the answer?

Haemophilus parainfluenzae requires V factor only, while H. influenzae also needs haemin.

Why is "Factor X (haemin) only" not the answer?

Haemophilus ducreyi, the cause of chancroid, requires X factor only.

Why is "Cysteine and iron salts" not the answer?

Cysteine and iron salts are the requirements of Legionella on buffered charcoal yeast extract agar.

What concept does this question test?

It tests Bacteriology: Haemophilus influenzae in Microbiology. Sheep blood agar lacks free V factor because of NADase, but S. aureus excretes NAD, which supports growth of satellite colonies. H. ducreyi needs X only, H. parainfluenzae needs V only, and Legionella requires cysteine and iron.

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 Bacteriology: Haemophilus influenzae?

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.