Mirizzi syndrome.

Remember this is when you get common hepatic duct obstruction due to external compression or impacted stone in the cystic duct vs Hartmann’s pouch. It can present exactly like acute cholecystitis or ascending cholangitis but intervention often leads to laprascopic injury to the biliary system. It is rare and if CT is negative, MRCP is the diagnostic modality of choice. ERCP or surgery once the diagnosis is made and these patients are thought to be at higher risk of gallbladder cancer.