Dental materials and radiology basics: delayed expansion of zinc-containing amalgam: practice question

A Dentistry 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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Dentistry · Dental materials and radiology basics: delayed expansion of zinc-containing amalgam

Sample item

A zinc-containing low-copper amalgam is condensed while the field is contaminated with saliva. Several days later the patient reports pain and the restoration stands proud of the cavity margin. What best explains this?

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

The answer and the reasoning

A. Hydrogen gas released by zinc reacting with moisture

Zinc in an amalgam alloy reacts with water from saliva or blood to liberate hydrogen gas inside the set mass. The gas causes delayed expansion that appears after days and can push the restoration out and cause pain. Strict moisture control, or zinc-free alloys, prevents it.

Option by option

  • A. Hydrogen gas released by zinc reacting with moisture (correct)

    Correct. Hydrogen from zinc and water produces delayed expansion after several days.

  • B. Creep of the tin-mercury phase under occlusal load

    Creep is a slow marginal deformation over months and years, not an expansion in days.

  • C. Excess mercury leaching and forming extra gamma-1 phase

    Mercury content affects strength and setting, but it does not produce late gas-driven expansion.

  • D. Greater thermal expansion of amalgam than of dentine

    Thermal expansion differences cause marginal percolation with temperature change, not progressive protrusion.

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 Dentistry question

What is the answer to this Dentistry question?

The best answer is A, Hydrogen gas released by zinc reacting with moisture. Zinc in an amalgam alloy reacts with water from saliva or blood to liberate hydrogen gas inside the set mass.

Why is "Creep of the tin-mercury phase under occlusal load" not the answer?

Creep is a slow marginal deformation over months and years, not an expansion in days.

Why is "Excess mercury leaching and forming extra gamma-1 phase" not the answer?

Mercury content affects strength and setting, but it does not produce late gas-driven expansion.

Why is "Greater thermal expansion of amalgam than of dentine" not the answer?

Thermal expansion differences cause marginal percolation with temperature change, not progressive protrusion.

What concept does this question test?

It tests Dental materials and radiology basics: delayed expansion of zinc-containing amalgam in Dentistry. The gas causes delayed expansion that appears after days and can push the restoration out and cause pain. Strict moisture control, or zinc-free alloys, prevents it.

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 Dental materials and radiology basics: delayed expansion of zinc-containing amalgam?

Learn the dental formula, eruption ages and tooth structure once, as tables. Then use the flashcards and the other Dentistry 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.