Tuberculosis diagnosis: MGIT liquid culture: 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 · Tuberculosis diagnosis: MGIT liquid culture

Sample item

In the Mycobacteria Growth Indicator Tube (MGIT) system, how is mycobacterial growth in the liquid medium detected?

ARelease of radioactive carbon dioxide from labelled palmitate is countedBOxygen consumption by growing bacteria unquenches a fluorescent compound seen under UV lightCTurbidity is read at 600 nm after 24 hoursDA phenol red indicator turns yellow as the pH falls

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

The answer and the reasoning

B. Oxygen consumption by growing bacteria unquenches a fluorescent compound seen under UV light

MGIT tubes contain modified Middlebrook 7H9 broth with a fluorescent compound quenched by oxygen. Growing mycobacteria consume oxygen, removing the quenching so the tube fluoresces under UV light, and instruments flag positive tubes automatically. It gives faster results than Lowenstein-Jensen culture and is used for drug susceptibility testing.

Option by option

A. Release of radioactive carbon dioxide from labelled palmitate is counted
Radiometric detection describes the older BACTEC 460 system, not MGIT.
B. Oxygen consumption by growing bacteria unquenches a fluorescent compound seen under UV light (correct)
Correct. Oxygen depletion by growing bacilli produces fluorescence.
C. Turbidity is read at 600 nm after 24 hours
Mycobacteria grow too slowly for turbidity to be read reliably at 24 hours.
D. A phenol red indicator turns yellow as the pH falls
Phenol red pH indicators are used in sugar fermentation tests, not in MGIT detection.

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, Oxygen consumption by growing bacteria unquenches a fluorescent compound seen under UV light. MGIT tubes contain modified Middlebrook 7H9 broth with a fluorescent compound quenched by oxygen.

Why is "Release of radioactive carbon dioxide from labelled palmitate is coun…" not the answer?

Radiometric detection describes the older BACTEC 460 system, not MGIT.

Why is "Turbidity is read at 600 nm after 24 hours" not the answer?

Mycobacteria grow too slowly for turbidity to be read reliably at 24 hours.

Why is "A phenol red indicator turns yellow as the pH falls" not the answer?

Phenol red pH indicators are used in sugar fermentation tests, not in MGIT detection.

What concept does this question test?

It tests Tuberculosis diagnosis: MGIT liquid culture in Microbiology. Growing mycobacteria consume oxygen, removing the quenching so the tube fluoresces under UV light, and instruments flag positive tubes automatically. It gives faster results than Lowenstein-Jensen culture and is used for drug susceptibility testing.

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 Tuberculosis diagnosis: MGIT liquid culture?

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.