Immunology: patch test for contact dermatitis: 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 · Immunology: patch test for contact dermatitis

Sample item

A woman has recurrent eczematous dermatitis under the clasp of her watch. Small amounts of suspected allergens are applied under occlusion on her back, and the sites are examined after removal. When are positive reactions read, and which cells mediate them?

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

The answer and the reasoning

A. At 48 to 72 hours; sensitised T lymphocytes

Allergic contact dermatitis, for example to nickel, is a type IV hypersensitivity in which sensitised T cells react with hapten-modified skin proteins. The patch test is read at 48 hours and again at 72 to 96 hours, showing erythema, papules or vesicles at the site.

Option by option

  • A. At 48 to 72 hours; sensitised T lymphocytes (correct)

    Correct. Patch tests assess delayed-type hypersensitivity, read at about 2 to 4 days.

  • B. At 1 week; complement-fixing IgM

    The patch test does not rely on complement-fixing IgM or a one-week reading.

  • C. At 4 to 6 hours; immune complexes

    A reaction at 4 to 6 hours with immune complexes describes the Arthus reaction.

  • D. At 15 to 20 minutes; IgE on mast cells

    Reactions at 15 to 20 minutes are IgE-mediated immediate wheals, seen in prick tests.

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.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is A, At 48 to 72 hours; sensitised T lymphocytes. Allergic contact dermatitis, for example to nickel, is a type IV hypersensitivity in which sensitised T cells react with hapten-modified skin proteins.

Why is "At 1 week; complement-fixing IgM" not the answer?

The patch test does not rely on complement-fixing IgM or a one-week reading.

Why is "At 4 to 6 hours; immune complexes" not the answer?

A reaction at 4 to 6 hours with immune complexes describes the Arthus reaction.

Why is "At 15 to 20 minutes; IgE on mast cells" not the answer?

Reactions at 15 to 20 minutes are IgE-mediated immediate wheals, seen in prick tests.

What concept does this question test?

It tests Immunology: patch test for contact dermatitis in Microbiology. The patch test is read at 48 hours and again at 72 to 96 hours, showing erythema, papules or vesicles at the site.

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 Immunology: patch test for contact dermatitis?

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.