This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: G.I.T, Subject name: Pharmacology
Colonoscopy performed on a 25-year-old woman with eating disorder showed dark brown to black pigmentary deposit in the lining of the large intestine. Histopathology of biopsy revealed pigment laden macrophages within the lamina propria. The woman on probing revealed use of laxatives for the 9 months to lose weight. What could be the probable laxative agent that could have caused these findings? 
Select and explain. 
A. Senna, B. Sorbitol, C. Castor oil, D. Bisacodyl
A: Senna
Answer: A: Senna 
Explanation: Ans. a. SennaRef: Goodman Gilman 12th e/P 1334Senna and cascara can cause melanotic pigmentation of colonic mucosa known as melanosis coli.