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: Pharmacology
[{'content': 'Melanosis coli is caused by \nSelect and explain. \nA. Bisacodyl, B. Senna, C. Magnesium sulphate, D. Lactulose', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Senna and cascara are anthraquinone derivatives, which are used as stimulant purgatives. On chronic use they can lead to melanosis coli.', 'role': 'assistant'}]