Environmental health: biochemical oxygen 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: biochemical oxygen demand

Sample item

A river sample downstream of a town has a five-day BOD at 20 degrees Celsius of 25 mg/L, while an upstream sample is 2 mg/L. What does this finding indicate?

APresence of heavy metals onlyBHigh level of dissolved oxygen and clean waterCHeavy load of biodegradable organic pollution from sewageDContamination with chemicals that are not biodegradable

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

The answer and the reasoning

C. Heavy load of biodegradable organic pollution from sewage

BOD is the oxygen consumed by micro-organisms in oxidising biodegradable organic matter over five days at 20 degrees Celsius. The higher the BOD, the greater the organic load and the lower the dissolved oxygen left for aquatic life. A very clean river has a BOD of about 1 to 2 mg/L, while untreated sewage is much higher.

Option by option

A. Presence of heavy metals only
Heavy metals are not oxidised by micro-organisms and do not raise BOD.
B. High level of dissolved oxygen and clean water
A high BOD lowers dissolved oxygen and signals pollution, not clean water.
C. Heavy load of biodegradable organic pollution from sewage (correct)
Correct. A high BOD reflects a large biodegradable organic load.
D. Contamination with chemicals that are not biodegradable
Non-biodegradable chemicals would not raise BOD but would be measured by COD.

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, Heavy load of biodegradable organic pollution from sewage. BOD is the oxygen consumed by micro-organisms in oxidising biodegradable organic matter over five days at 20 degrees Celsius.

Why is "Presence of heavy metals only" not the answer?

Heavy metals are not oxidised by micro-organisms and do not raise BOD.

Why is "High level of dissolved oxygen and clean water" not the answer?

A high BOD lowers dissolved oxygen and signals pollution, not clean water.

Why is "Contamination with chemicals that are not biodegradable" not the answer?

Non-biodegradable chemicals would not raise BOD but would be measured by COD.

What concept does this question test?

It tests Environmental health: biochemical oxygen demand in PSM. The higher the BOD, the greater the organic load and the lower the dissolved oxygen left for aquatic life. A very clean river has a BOD of about 1 to 2 mg/L, while untreated sewage is much higher.

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: biochemical oxygen 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.