Bacteriology: anaerobes: 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: anaerobes

Sample item

A farmer has a deep contaminated leg wound with severe pain, crepitus and a brownish discharge. Gram stain shows large Gram-positive bacilli without spores. On egg-yolk agar, colonies show opalescence that is inhibited by antiserum (Nagler reaction). Which organism and toxin?

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

The answer and the reasoning

C. Clostridium perfringens, alpha toxin (lecithinase)

Clostridium perfringens produces alpha toxin, a lecithinase (phospholipase C), which causes gas gangrene by destroying cell membranes. The Nagler reaction shows opalescence on egg-yolk agar which is neutralised by specific antitoxin. Treatment is urgent debridement, penicillin with clindamycin and sometimes hyperbaric oxygen.

Option by option

  • A. Clostridium botulinum, botulinum toxin

    C. botulinum causes flaccid paralysis.

  • B. Bacillus cereus, emetic toxin

    Bacillus cereus causes food poisoning and is a spore-forming aerobe.

  • C. Clostridium perfringens, alpha toxin (lecithinase) (correct)

    Correct. Lecithinase activity in the Nagler reaction identifies Clostridium perfringens.

  • D. Clostridium tetani, tetanospasmin

    Tetanus causes spasms through tetanospasmin and does not produce gas gangrene.

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, Clostridium perfringens, alpha toxin (lecithinase). Clostridium perfringens produces alpha toxin, a lecithinase (phospholipase C), which causes gas gangrene by destroying cell membranes.

Why is "Clostridium botulinum, botulinum toxin" not the answer?

C. botulinum causes flaccid paralysis.

Why is "Bacillus cereus, emetic toxin" not the answer?

Bacillus cereus causes food poisoning and is a spore-forming aerobe.

Why is "Clostridium tetani, tetanospasmin" not the answer?

Tetanus causes spasms through tetanospasmin and does not produce gas gangrene.

What concept does this question test?

It tests Bacteriology: anaerobes in Microbiology. The Nagler reaction shows opalescence on egg-yolk agar which is neutralised by specific antitoxin. Treatment is urgent debridement, penicillin with clindamycin and sometimes hyperbaric oxygen.

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: anaerobes?

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.