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
Black bile stones are composed of: 
Select and explain. 
A. Calcium bilirubinate stones, B. Cholesterol monohydrate stones, C. Calcium monohydrate stones, D. Cholesterol bilirubinate stones
A: Calcium bilirubinate stones
Answer: A: Calcium bilirubinate stones 
Explanation: There are cholesterol stones and pigment stones. Cholesterol gallstones usually contain >50% cholesterol monohydrate. Pigment stones are composed primarily of calcium bilirubinate; they contain <20% cholesterol. Reference: Harrisons Principles of Internal Medicine, 18th Edition, Pages 2616-17