This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gastrointestinal tract, Subject name: Pharmacology
[{'content': 'Which of the following drug cause the following effect? (colonoscopic view) \nSelect and explain. \nA. Senna, B. Bisacodyl, C. Bismuth, D. Aluminium Hydroxide', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Bismuth cause tongue and stool discoloration. Senna(irritant purgative) cause colon pigmentation - melanosis coli. ref: katzung 12th ed', 'role': 'assistant'}]