Staphylococcus aureus: protein A: 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 · Staphylococcus aureus: protein A

Sample item

In a coagglutination test for rapid detection of bacterial antigen in cerebrospinal fluid, Staphylococcus aureus Cowan I cells are coated with specific antibody. Which staphylococcal component binds the antibody, and where?

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

The answer and the reasoning

B. Protein A binding the Fc region of IgG

Protein A in the S. aureus cell wall binds the Fc portion of IgG, leaving the Fab regions free to bind antigen. This property is used in coagglutination, and in vivo it impairs opsonisation and phagocytosis. Clumping factor (bound coagulase) binds fibrinogen.

Option by option

  • A. Protein A binding the Fab region of IgG

    Binding through Fab would block antigen recognition, and protein A does not do this.

  • B. Protein A binding the Fc region of IgG (correct)

    Correct. Protein A attaches to the Fc region and leaves antigen-binding sites exposed.

  • C. Clumping factor binding fibrinogen

    Clumping factor binds fibrinogen, not antibody.

  • D. Teichoic acid binding the Fc region of IgG

    Teichoic acid does not bind the Fc region of IgG.

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 B, Protein A binding the Fc region of IgG. Protein A in the S.

Why is "Protein A binding the Fab region of IgG" not the answer?

Binding through Fab would block antigen recognition, and protein A does not do this.

Why is "Clumping factor binding fibrinogen" not the answer?

Clumping factor binds fibrinogen, not antibody.

Why is "Teichoic acid binding the Fc region of IgG" not the answer?

Teichoic acid does not bind the Fc region of IgG.

What concept does this question test?

It tests Staphylococcus aureus: protein A in Microbiology. aureus cell wall binds the Fc portion of IgG, leaving the Fab regions free to bind antigen. This property is used in coagglutination, and in vivo it impairs opsonisation and phagocytosis. Clumping factor (bound coagulase) binds fibrinogen.

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 Staphylococcus aureus: protein A?

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.