This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: G.I.T, Subject name: Anatomy
Which of the following calculi are rare in gallbladder, but common in common bile duct 
Select and explain. 
A. Cholesterol stone, B. Brown stone, C. Black stone, D. None
B: Brown stone
Answer: B: Brown stone 
Explanation: Primary CBD Stone Formed within the biliary tract. Associated with biliary stasis and infection Most commonly seen in Asian population Soft, Friable, light brown stones or sludge in the common duct In Gallbladder mostly cholesterol stones are formed that migrate through cystic duct into CBD known as Secondary CBD stone Ref: Sabiston 20th edition Pgno :1494