Culture media: Haemophilus growth factors: 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 · Culture media: Haemophilus growth factors

Sample item

Gram-negative coccobacilli from sputum fail to grow on blood agar but grow on chocolate agar and form small colonies around a Staphylococcus aureus streak on blood agar. Which growth factors does this organism need?

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

The answer and the reasoning

C. Both X (haemin) and V (NAD) factors

Haemophilus influenzae needs haemin (X factor) and nicotinamide adenine dinucleotide (V factor). Chocolate agar releases both from lysed red cells, and S. aureus secretes NAD, which gives the satellitism phenomenon on blood agar. H. ducreyi needs only X factor and H. parainfluenzae only V factor.

Option by option

  • A. X factor only

    X factor alone is the requirement of H. ducreyi, which would not satellite around staphylococci.

  • B. V factor only

    V factor alone is the requirement of H. parainfluenzae, which does not match the vignette.

  • C. Both X (haemin) and V (NAD) factors (correct)

    Correct. H. influenzae needs both factors, hence satellitism.

  • D. Neither factor, but cysteine and iron

    Cysteine and iron are requirements for Francisella tularensis and Legionella, not Haemophilus.

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 C, Both X (haemin) and V (NAD) factors. Haemophilus influenzae needs haemin (X factor) and nicotinamide adenine dinucleotide (V factor).

Why is "X factor only" not the answer?

X factor alone is the requirement of H. ducreyi, which would not satellite around staphylococci.

Why is "V factor only" not the answer?

V factor alone is the requirement of H. parainfluenzae, which does not match the vignette.

Why is "Neither factor, but cysteine and iron" not the answer?

Cysteine and iron are requirements for Francisella tularensis and Legionella, not Haemophilus.

What concept does this question test?

It tests Culture media: Haemophilus growth factors in Microbiology. Chocolate agar releases both from lysed red cells, and S. aureus secretes NAD, which gives the satellitism phenomenon on blood agar. H. ducreyi needs only X factor and H. parainfluenzae only V factor.

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 Culture media: Haemophilus growth factors?

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.