Bacteriology: Bordetella pertussis: 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: Bordetella pertussis

Sample item

A 4-month-old unimmunised infant has paroxysms of cough ending in an inspiratory whoop with post-tussive vomiting. Total leucocyte count is 38,000 per microlitre with 75% lymphocytes. Which toxin action best explains the lymphocytosis?

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

The answer and the reasoning

C. Pertussis toxin ADP-ribosylates Gi, so lymphocytes cannot leave the blood

Pertussis toxin ADP-ribosylates the Gi regulatory protein and impairs chemokine-driven extravasation of lymphocytes, so they accumulate in the blood, producing a marked absolute lymphocytosis without a lymphoid reaction. The other toxins contribute to local damage and impaired phagocyte killing but do not cause lymphocytosis.

Option by option

  • A. Adenylate cyclase toxin kills lymphocytes in the marrow and triggers compensatory release

    Adenylate cyclase toxin enters phagocytes and raises cAMP to impair their killing; it is not a marrow toxin.

  • B. Dermonecrotic toxin causes lymphocyte proliferation in the spleen

    Dermonecrotic toxin causes local vasoconstriction and tissue necrosis and has no role in lymphocytosis.

  • C. Pertussis toxin ADP-ribosylates Gi, so lymphocytes cannot leave the blood (correct)

    Correct. Blocked lymphocyte homing produces lymphocytosis with an otherwise normal-looking lymphocyte population.

  • D. Tracheal cytotoxin destroys ciliated cells and releases lymphocytes from the mucosa

    Tracheal cytotoxin damages ciliated epithelium and impairs mucociliary clearance but does not mobilise lymphocytes.

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, Pertussis toxin ADP-ribosylates Gi, so lymphocytes cannot leave the blood. Pertussis toxin ADP-ribosylates the Gi regulatory protein and impairs chemokine-driven extravasation of lymphocytes, so they accumulate in the blood, producing a marked absolute lymphocytosis without a lymphoid reaction.

Why is "Adenylate cyclase toxin kills lymphocytes in the marrow and triggers…" not the answer?

Adenylate cyclase toxin enters phagocytes and raises cAMP to impair their killing; it is not a marrow toxin.

Why is "Dermonecrotic toxin causes lymphocyte proliferation in the spleen" not the answer?

Dermonecrotic toxin causes local vasoconstriction and tissue necrosis and has no role in lymphocytosis.

Why is "Tracheal cytotoxin destroys ciliated cells and releases lymphocytes f…" not the answer?

Tracheal cytotoxin damages ciliated epithelium and impairs mucociliary clearance but does not mobilise lymphocytes.

What concept does this question test?

It tests Bacteriology: Bordetella pertussis in Microbiology. The other toxins contribute to local damage and impaired phagocyte killing but do not cause lymphocytosis.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: Bordetella pertussis?

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.