Concept of causation: necessary and sufficient causes: 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 causation: necessary and sufficient causes

Sample item

Only a minority of people infected with Mycobacterium tuberculosis develop clinical tuberculosis, but the disease cannot occur without the bacillus. In causal terminology, the bacillus is:

AA necessary but not sufficient causeBA sufficient but not necessary causeCNeither necessary nor sufficientDBoth necessary and sufficient

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

The answer and the reasoning

A. A necessary but not sufficient cause

A necessary cause must be present for the disease to occur, and a sufficient cause always produces the disease. Tuberculosis never occurs without the bacillus, so it is necessary, but most infected people never develop disease, so it is not sufficient. Host and environmental factors such as malnutrition and HIV infection are also required.

Option by option

A. A necessary but not sufficient cause (correct)
Correct. The bacillus is required, but other factors determine whether disease follows.
B. A sufficient but not necessary cause
A sufficient cause would always produce disease, which does not occur here.
C. Neither necessary nor sufficient
Because disease never occurs without it, the organism is clearly necessary.
D. Both necessary and sufficient
If it were also sufficient, every infected person would become ill.

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, A necessary but not sufficient cause. A necessary cause must be present for the disease to occur, and a sufficient cause always produces the disease.

Why is "A sufficient but not necessary cause" not the answer?

A sufficient cause would always produce disease, which does not occur here.

Why is "Neither necessary nor sufficient" not the answer?

Because disease never occurs without it, the organism is clearly necessary.

Why is "Both necessary and sufficient" not the answer?

If it were also sufficient, every infected person would become ill.

What concept does this question test?

It tests Concept of causation: necessary and sufficient causes in PSM. Tuberculosis never occurs without the bacillus, so it is necessary, but most infected people never develop disease, so it is not sufficient. Host and environmental factors such as malnutrition and HIV infection are also required.

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 causation: necessary and sufficient causes?

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.