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
Which of the following drug cause the following effect? (colonoscopic view) 
Select and explain. 
A. Senna, B. Bisacodyl, C. Bismuth, D. Aluminium Hydroxide
A: Senna
Answer: A: Senna 
Explanation: Bismuth cause tongue and stool discoloration. Senna(irritant purgative) cause colon pigmentation - melanosis coli. ref: katzung 12th ed