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
The prognosis of bleaching of a discoloured tooth is good in all the given conditions except: 
Select and explain. 
A. 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.
C: Discolouration due to metallic salts.
Answer: C: Discolouration due to metallic salts. 
Explanation: The prognosis for bleaching of a discolored pulpless tooth is good when the discoloration is due to products of pulp decomposition, food debris, or chromogenic bacteria that gain access to the dentinal tubules. When the discoloration is due to metallic salts, bleaching is less successful.