Concept of health and disease: Koch's postulates: 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 · Concept of health and disease: Koch's postulates

Sample item

Mycobacterium leprae cannot be grown on artificial culture media. Which of Koch's original postulates can therefore not be fulfilled for leprosy?

AGrowth of the organism in pure culture outside the hostBReproduction of the disease by inoculation into a susceptible animalCRecovery of the organism again from the experimentally infected hostDDemonstration of the organism in the lesions of the disease

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

The answer and the reasoning

A. Growth of the organism in pure culture outside the host

Koch's postulates require the organism to be found in every case, grown in pure culture, able to reproduce the disease on inoculation into a susceptible host, and recovered again. M. leprae fails the pure-culture step because it does not grow on artificial media. This is why Koch's postulates are not applicable to every infection.

Option by option

A. Growth of the organism in pure culture outside the host (correct)
Correct. Failure to grow on artificial media violates the pure-culture postulate.
B. Reproduction of the disease by inoculation into a susceptible animal
Armadillos develop disease after inoculation, so this step can be met.
C. Recovery of the organism again from the experimentally infected host
Bacilli multiply in the armadillo and the mouse footpad, so the organism can be recovered again from inoculated hosts.
D. Demonstration of the organism in the lesions of the disease
Bacilli are readily shown in smears from lepromatous lesions, so this step can be met.

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 A, Growth of the organism in pure culture outside the host. Koch's postulates require the organism to be found in every case, grown in pure culture, able to reproduce the disease on inoculation into a susceptible host, and recovered again.

Why is "Reproduction of the disease by inoculation into a susceptible animal" not the answer?

Armadillos develop disease after inoculation, so this step can be met.

Why is "Recovery of the organism again from the experimentally infected host" not the answer?

Bacilli multiply in the armadillo and the mouse footpad, so the organism can be recovered again from inoculated hosts.

Why is "Demonstration of the organism in the lesions of the disease" not the answer?

Bacilli are readily shown in smears from lepromatous lesions, so this step can be met.

What concept does this question test?

It tests Concept of health and disease: Koch's postulates in PSM. M. leprae fails the pure-culture step because it does not grow on artificial media. This is why Koch's postulates are not applicable to every infection.

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 Concept of health and disease: Koch's postulates?

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.