Environmental health: chlorine demand: 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: chlorine demand

Sample item

A health inspector must disinfect a village well and first needs to find how much bleaching powder the well water requires. Which instrument is used for this purpose?

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

The answer and the reasoning

A. Horrock's apparatus

Horrock's apparatus is used to determine the chlorine demand of water, that is, the amount of bleaching powder needed to give a free residual chlorine after contact time. It uses a series of cups with increasing doses and a starch-iodide indicator. The orthotolidine method measures residual chlorine afterwards.

Option by option

  • A. Horrock's apparatus (correct)

    Correct. Horrock's apparatus finds the bleaching powder dose needed.

  • B. Orthotolidine comparator

    The orthotolidine method estimates residual chlorine, not the dose required.

  • C. Lux meter

    A lux meter measures illumination.

  • D. Sling psychrometer

    A sling psychrometer measures air humidity.

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 PSM question

What is the answer to this PSM question?

The best answer is A, Horrock's apparatus. Horrock's apparatus is used to determine the chlorine demand of water, that is, the amount of bleaching powder needed to give a free residual chlorine after contact time.

Why is "Orthotolidine comparator" not the answer?

The orthotolidine method estimates residual chlorine, not the dose required.

Why is "Lux meter" not the answer?

A lux meter measures illumination.

Why is "Sling psychrometer" not the answer?

A sling psychrometer measures air humidity.

What concept does this question test?

It tests Environmental health: chlorine demand in PSM. It uses a series of cups with increasing doses and a starch-iodide indicator. The orthotolidine method measures residual chlorine afterwards.

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: chlorine demand?

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.