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
Pigment stone is composed of? 
Select and explain. 
A. Ca bilirubinate, B. Ca phosphate, C. Ca carbonate, D. Ca gluconate
A: Ca bilirubinate
Answer: A: Ca bilirubinate 
Explanation: 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.