This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gallbladder, Subject name: Surgery
[{'content': 'Gallstones do not contain: \nSelect and explain. \nA. Oxalate, B. Cholesterol, C. Phosphate, D. Carbonate', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Gallstones In the USA and Europe, 80% are cholesterol or mixed stones, whereas in Asia, 80% are pigment stones Cholesterol or mixed stones - 51-99% pure cholesterol Pigment stone contains <30% cholesterol Black stones - Insoluble bilirubin polymer + calcium phosphate + calcium bicarbonate Brown pigment stones- calcium bilirubinate+ calcium palmitate + calcium stearate+ cholesterol', 'role': 'assistant'}]