Bacteriology: Mycoplasma pneumoniae: 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: Mycoplasma pneumoniae

Sample item

A 19-year-old college student has a 10-day dry cough, low-grade fever and headache. The chest radiograph shows patchy infiltrates out of proportion to the clinical findings, and there was no improvement on amoxicillin. Gram stain of sputum shows no organisms. Which property explains the failure of amoxicillin?

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

The answer and the reasoning

C. Absence of a peptidoglycan cell wall

Mycoplasma pneumoniae is the smallest free-living bacterium and lacks a cell wall, so beta-lactams that inhibit peptidoglycan synthesis are ineffective and the organism does not stain on Gram stain. Macrolides, doxycycline or respiratory fluoroquinolones are active. Cold agglutinins and the Eaton agent history are associated with this atypical pneumonia.

Option by option

  • A. A thick mycolic acid outer layer

    Mycolic acids are present in mycobacteria, not in Mycoplasma.

  • B. Production of beta-lactamase

    Beta-lactamase destroys the drug in staphylococci and many Gram-negative bacilli; Mycoplasma has no such need because it lacks the target.

  • C. Absence of a peptidoglycan cell wall (correct)

    Correct. With no peptidoglycan there is no target for amoxicillin.

  • D. Altered penicillin-binding protein 2a

    PBP2a confers methicillin resistance in MRSA and is irrelevant here.

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.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, Absence of a peptidoglycan cell wall. Mycoplasma pneumoniae is the smallest free-living bacterium and lacks a cell wall, so beta-lactams that inhibit peptidoglycan synthesis are ineffective and the organism does not stain on Gram stain.

Why is "A thick mycolic acid outer layer" not the answer?

Mycolic acids are present in mycobacteria, not in Mycoplasma.

Why is "Production of beta-lactamase" not the answer?

Beta-lactamase destroys the drug in staphylococci and many Gram-negative bacilli; Mycoplasma has no such need because it lacks the target.

Why is "Altered penicillin-binding protein 2a" not the answer?

PBP2a confers methicillin resistance in MRSA and is irrelevant here.

What concept does this question test?

It tests Bacteriology: Mycoplasma pneumoniae in Microbiology. Macrolides, doxycycline or respiratory fluoroquinolones are active. Cold agglutinins and the Eaton agent history are associated with this atypical pneumonia.

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: Mycoplasma pneumoniae?

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.