Agent factors: pathogenicity and virulence: 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 · Agent factors: pathogenicity and virulence

Sample item

An outbreak investigation finds that of 200 persons infected with an organism, 80 developed clinical disease and 8 of these died. Taking virulence as the case fatality among clinical cases, what are the pathogenicity and the virulence?

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

The answer and the reasoning

B. Pathogenicity 40 percent, virulence 10 percent

Pathogenicity is the proportion of infected people who develop clinical disease: 80 divided by 200 is 40 percent. Virulence is the proportion of clinical cases that are severe or fatal: 8 divided by 80 is 10 percent. The denominators differ, so the two should not be mixed.

Option by option

  • A. Pathogenicity 4 percent, virulence 10 percent

    Dividing deaths by all infected as 4 percent for pathogenicity confuses the numerator, since pathogenicity counts clinical cases.

  • B. Pathogenicity 40 percent, virulence 10 percent (correct)

    Correct. 80/200 = 40 percent and 8/80 = 10 percent.

  • C. Pathogenicity 10 percent, virulence 40 percent

    The two values have been swapped.

  • D. Pathogenicity 40 percent, virulence 4 percent

    Dividing deaths by all infected (8/200 = 4 percent) uses the wrong denominator for virulence.

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 B, Pathogenicity 40 percent, virulence 10 percent. Pathogenicity is the proportion of infected people who develop clinical disease: 80 divided by 200 is 40 percent.

Why is "Pathogenicity 4 percent, virulence 10 percent" not the answer?

Dividing deaths by all infected as 4 percent for pathogenicity confuses the numerator, since pathogenicity counts clinical cases.

Why is "Pathogenicity 10 percent, virulence 40 percent" not the answer?

The two values have been swapped.

Why is "Pathogenicity 40 percent, virulence 4 percent" not the answer?

Dividing deaths by all infected (8/200 = 4 percent) uses the wrong denominator for virulence.

What concept does this question test?

It tests Agent factors: pathogenicity and virulence in PSM. Virulence is the proportion of clinical cases that are severe or fatal: 8 divided by 80 is 10 percent. The denominators differ, so the two should not be mixed.

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 Agent factors: pathogenicity and virulence?

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.