Corynebacterium diphtheriae: Schick test: 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 · Corynebacterium diphtheriae: Schick test

Sample item

A Schick test is done in a child by injecting diphtheria toxin intradermally into one forearm and heat-inactivated toxin into the other. After 5 days, the test arm shows 2 cm of erythema and induration, and the control arm shows none. How should this be interpreted?

AImmune, with protective antitoxin levelsBPseudoreaction caused by hypersensitivity to toxin proteinCSusceptible, with inadequate circulating antitoxinDCombined reaction indicating both susceptibility and hypersensitivity

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

The answer and the reasoning

C. Susceptible, with inadequate circulating antitoxin

In the Schick test, circulating antitoxin neutralises the injected toxin, so an immune person shows no reaction. A positive reaction at the test site with a negative control means no protective antitoxin, so the child is susceptible. Reactions at both sites indicate a pseudoreaction due to protein hypersensitivity.

Option by option

A. Immune, with protective antitoxin levels
An immune child would neutralise the toxin and show no reaction in the test arm.
B. Pseudoreaction caused by hypersensitivity to toxin protein
A pseudoreaction would appear on both arms, since heated toxin retains protein antigens.
C. Susceptible, with inadequate circulating antitoxin (correct)
Correct. A positive test arm with a negative control arm indicates susceptibility to diphtheria.
D. Combined reaction indicating both susceptibility and hypersensitivity
A combined reaction would show a reaction on the control arm too, which did not occur here.

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 C, Susceptible, with inadequate circulating antitoxin. In the Schick test, circulating antitoxin neutralises the injected toxin, so an immune person shows no reaction.

Why is "Immune, with protective antitoxin levels" not the answer?

An immune child would neutralise the toxin and show no reaction in the test arm.

Why is "Pseudoreaction caused by hypersensitivity to toxin protein" not the answer?

A pseudoreaction would appear on both arms, since heated toxin retains protein antigens.

Why is "Combined reaction indicating both susceptibility and hypersensitivity" not the answer?

A combined reaction would show a reaction on the control arm too, which did not occur here.

What concept does this question test?

It tests Corynebacterium diphtheriae: Schick test in Microbiology. A positive reaction at the test site with a negative control means no protective antitoxin, so the child is susceptible. Reactions at both sites indicate a pseudoreaction due to protein hypersensitivity.

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 Corynebacterium diphtheriae: Schick test?

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.