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': 'The prognosis of bleaching of a discoloured tooth is good in all the given conditions except: \nSelect and explain. \nA. Discolouration due to products of pulp decomposition., B. Discolouration due to food debris., C. Discolouration due to metallic salts., D. Discolouration due to chromogenic bacteria.', 'role': 'user'}, {'content': 'Answer: C \nExplanation: The prognosis for bleaching of a discolored\xa0pulpless tooth is good when the discoloration is\xa0due to products of pulp decomposition, food debris, or chromogenic bacteria that gain access to the\xa0dentinal tubules. When the discoloration is due to metallic salts, bleaching is less successful.', 'role': 'assistant'}]