This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Liver & Biliary Tract, Subject name: Pathology
[{'content': 'Pigment stone is composed of? \nSelect and explain. \nA. Ca bilirubinate, B. Ca phosphate, C. Ca carbonate, D. Ca gluconate', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. (a) Ca bilirubinate(Ref: Robbins 9th/pg 877)Two main types of gallstones:Cholesterol stones (most common type in West) containing crystalline cholesterol monohydrate.Pigment stones composed mainly of calcium bilirubinate; contain <20% cholesterol and are classified into "black" & "brown" types, the latter forming secondary to chronic biliary infection.', 'role': 'assistant'}]