Environmental health: bacteriological examination of water: 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: bacteriological examination of water

Sample item

In the multiple tube method for the presumptive coliform count, which finding in a tube of MacConkey broth inoculated with water and incubated at 37 degrees Celsius is considered positive?

AFormation of a surface pellicleBClear broth with no colour changeCAcid and gas productionDTurbidity alone without gas

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

The answer and the reasoning

C. Acid and gas production

Coliforms ferment lactose in MacConkey broth, producing acid (colour change) and gas collected in a Durham tube. The number of positive tubes is converted to the most probable number per 100 mL using standard tables. Presumptive positives are confirmed by tests for E. coli at 44 degrees Celsius.

Option by option

A. Formation of a surface pellicle
A surface pellicle is not the criterion in the presumptive test.
B. Clear broth with no colour change
A clear unchanged tube indicates no coliform growth.
C. Acid and gas production (correct)
Correct. Lactose fermentation gives acid and gas.
D. Turbidity alone without gas
Turbidity without gas does not show lactose fermentation with gas.

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, Acid and gas production. Coliforms ferment lactose in MacConkey broth, producing acid (colour change) and gas collected in a Durham tube.

Why is "Formation of a surface pellicle" not the answer?

A surface pellicle is not the criterion in the presumptive test.

Why is "Clear broth with no colour change" not the answer?

A clear unchanged tube indicates no coliform growth.

Why is "Turbidity alone without gas" not the answer?

Turbidity without gas does not show lactose fermentation with gas.

What concept does this question test?

It tests Environmental health: bacteriological examination of water in PSM. The number of positive tubes is converted to the most probable number per 100 mL using standard tables. Presumptive positives are confirmed by tests for E. coli at 44 degrees Celsius.

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 Environmental health: bacteriological examination of water?

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.