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: Surgery
Which of the following is an etiological factor in Mirizzi's syndrome? 
Select and explain. 
A. Gall bladder stone compressing common hepatic duct, B. Gall bladder carcinoma invading IVC, C. Gall bladder stone causing cholecystitis, D. Pancreatic carcinoma
A: Gall bladder stone compressing common hepatic duct
Answer: A: Gall bladder stone compressing common hepatic duct 
Explanation: Mirizzi's syndrome refers to obstruction of common hepatic duct resulting from compression by a gallstone impacted in the cystic duct. Two types of Mirizzi's syndrome are: Type I : Hepatic duct is compressed by a large stone impacted in the cystic duct or Haman's pouch. Type II: Calculus has eroded into the hepatic duct to produce a cholecystocholedochal fistula. Investigations: Ultrasonography reveals gallstones with a contracted gallbladder and moderate intrahepatic ductal dilatation with normal extrahepatic biliary anatomy.