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': 'When gold fused with porcelain is fired at very high temperature, greenish discoloration of cervical margin is due to: \nSelect and explain. \nA. High firing temperature, B. Disintegration of opaque layer, C. Presence of silver, D. Formation of excess oxides', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Discoloration\xa0of\xa0the\xa0porcelain\xa0near\xa0the\xa0cervical\xa0region\xa0of\xa0the metal-ceramic prosthesis has been reported to occur when a silver-containing alloy is used as the substrate. Color changes included\xa0green,\xa0yellow-green,\xa0yellow-orange, orange\xa0and brown\xa0hues. This\xa0discoloration\xa0 phenomenon\xa0has\xa0generally been called “greening.” It was likely caused by the colloidal dispersion of silver atoms entering body and incisal porcelain or the glazed surface from vapor transport or surface diffusion.\nThe extent of porcelain discoloration was most severe for higher-silver-content alloys, lighter shades, multiple firing procedures, higher temperatures, body porcelain in direct contact with the alloy, vacuum firing cycles\xa0and with certain porcelains\xa0containing lower\xa0opacifier\xa0and\xa0higher\xa0sodium contents.\nSilver is added to Au-Ag-Cu casting alloys to offset\xa0the reddish hue contributed by Cu.\nRef: Phillips 12 ed page no 413', 'role': 'assistant'}]