This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
[{'content': 'An 8 year old male patient complains of food lodgement with a history of previous restoration of the same tooth. Intra-oral examination reveals fractured Class II amalgam restoration with mandibular molar. Which of the following is not responsible for the fracture of restoration? \nSelect and explain. \nA. Poor cavity design resulting in an insufficient bulk of material across the isthmus can lead to failure of even a high-strength alloy, B. Avoiding moisture contamination of amalgam, C. Non-compliance of the child with instructions to refrain from biting down hard on the freshly placed amalgam, D. Thin edges of restoration', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Moisture contamination of an amalgam restoration can promote failure. If zinc is present in the alloy, it will react with water, and hydrogen gas will be formed. As this gas builds up within the amalgam, a significant delayed\xa0expansion\xa0 can\xa0 occur\xa0 and\xa0 may\xa0 cause\xa0 protrusion\xa0 of\xa0 the amalgam\xa0 from\xa0 the\xa0 cavity\xa0 preparation,\xa0 which\xa0 enhances the possibility of fracture at the margins. If the restoration is improperly finished by the dentist,\xa0a thin ledge of amalgam may be left that extends slightly over the enamel at the margins. These thin edges of such a brittle material cannot support the forces of mastication. In time they fracture, leaving an opening at the margins. Poor cavity design resulting in an insufficient bulk of material across the isthmus can lead to failure of even a high-strength alloy.\nUnlike a resin matrix composite, amalgam gains strength slowly over the first\xa024\xa0hours.\xa0Premature\xa0loading\xa0can\xa0result\xa0in\xa0minute fractures that are not apparent for weeks or even months. The use of a rapid-setting amalgam with a high 1-hour compressive\xa0 strength\xa0 should\xa0 be\xa0 considered\xa0 during\xa0 the treatment\xa0 of\xa0 a\xa0 pediatric\xa0 patient,\xa0 in\xa0 whom\xa0 compliance with instructions to refrain from biting down hard on the freshly placed amalgam is in question.', 'role': 'assistant'}]