Environmental health: Clostridium perfringens as an indicator: practice question

A PSM 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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PSM · Environmental health: Clostridium perfringens as an indicator

Sample item

A water sample grows spores of Clostridium perfringens but no E. coli. How should the result be interpreted?

ARecent fresh faecal contaminationBSafe water because E. coli is absentCRemote or past faecal contaminationDContamination only by industrial waste

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

The answer and the reasoning

C. Remote or past faecal contamination

E. coli indicates recent faecal pollution because it survives only a short time outside the gut. Clostridium perfringens spores persist for long periods, so their presence without E. coli suggests remote or intermittent contamination. Such water is still unsafe to assume potable.

Option by option

A. Recent fresh faecal contamination
Recent contamination would be indicated by E. coli.
B. Safe water because E. coli is absent
Absence of E. coli does not make water safe when spores indicate earlier faecal pollution.
C. Remote or past faecal contamination (correct)
Correct. Persistent spores point to past contamination.
D. Contamination only by industrial waste
Perfringens is a gut organism, not an industrial indicator.

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.

Questions about this PSM question

What is the answer to this PSM question?

The best answer is C, Remote or past faecal contamination. E.

Why is "Recent fresh faecal contamination" not the answer?

Recent contamination would be indicated by E. coli.

Why is "Safe water because E. coli is absent" not the answer?

Absence of E. coli does not make water safe when spores indicate earlier faecal pollution.

Why is "Contamination only by industrial waste" not the answer?

Perfringens is a gut organism, not an industrial indicator.

What concept does this question test?

It tests Environmental health: Clostridium perfringens as an indicator in PSM. coli indicates recent faecal pollution because it survives only a short time outside the gut. Clostridium perfringens spores persist for long periods, so their presence without E. coli suggests remote or intermittent contamination. Such water is still unsafe to assume potable.

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 Environmental health: Clostridium perfringens as an indicator?

Practise biostatistics with small calculations until sensitivity, predictive values and risk measures are automatic. Then use the flashcards and the other PSM 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.