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: Medicine
[{'content': 'Brown type gallstones are formed due to: \nSelect and explain. \nA. Hypercholesterolemia, B. Long standing hemolysis, C. Chronic biliary infection, D. Drugs', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Pigment stones are composed of calcium bilirubinate and are classified into "black" and "brown" types, the latter forming secondary to chronic biliary infection. Reference: Harrisons Principles of Internal Medicine, 18th Edition, Pages 2616-17', 'role': 'assistant'}]