General microbiology and sterilisation: chemical indicator for autoclave: 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 · General microbiology and sterilisation: chemical indicator for autoclave

Sample item

A technician places a sealed glass tube containing a red liquid inside an autoclave load as a quick check of conditions. After a complete cycle at 121 degrees C the liquid has turned green. What does this tube represent and what does the colour change show?

AA spore strip; viable spores were killedBA Browne's tube; the sterilising temperature was reachedCA Bowie-Dick test; air was removed from the loadDA thermocouple; the chamber pressure was maintained

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

The answer and the reasoning

B. A Browne's tube; the sterilising temperature was reached

Browne's tubes are chemical indicators whose liquid changes from red to green when the exposure for the stated temperature and time has been reached. They show that conditions were achieved but do not prove sterility the way a spore biological indicator does. The Bowie-Dick test is a separate air-removal check for pre-vacuum machines.

Option by option

A. A spore strip; viable spores were killed
A spore indicator contains bacterial spores that are cultured afterwards, and it does not change colour in the cycle.
B. A Browne's tube; the sterilising temperature was reached (correct)
Correct. The red to green change signals that the preset temperature-time exposure was attained.
C. A Bowie-Dick test; air was removed from the load
The Bowie-Dick test uses a folded towel pack with a heat-sensitive sheet to detect air leaks or residual air, not a tube of red liquid.
D. A thermocouple; the chamber pressure was maintained
A thermocouple is an electrical probe that records temperature, and it is not a colour 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 Microbiology question

What is the answer to this Microbiology question?

The best answer is B, A Browne's tube; the sterilising temperature was reached. Browne's tubes are chemical indicators whose liquid changes from red to green when the exposure for the stated temperature and time has been reached.

Why is "A spore strip; viable spores were killed" not the answer?

A spore indicator contains bacterial spores that are cultured afterwards, and it does not change colour in the cycle.

Why is "A Bowie-Dick test; air was removed from the load" not the answer?

The Bowie-Dick test uses a folded towel pack with a heat-sensitive sheet to detect air leaks or residual air, not a tube of red liquid.

Why is "A thermocouple; the chamber pressure was maintained" not the answer?

A thermocouple is an electrical probe that records temperature, and it is not a colour indicator.

What concept does this question test?

It tests General microbiology and sterilisation: chemical indicator for autoclave in Microbiology. They show that conditions were achieved but do not prove sterility the way a spore biological indicator does. The Bowie-Dick test is a separate air-removal check for pre-vacuum machines.

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 General microbiology and sterilisation: chemical indicator for autoclave?

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.