Which of the following are causes for cholecystitis wxcept?

acalculous cholecystitis is not an uncommon entity, but can be commonly missed.* It is common in patients who have undergone major surgeries, trauma, burns, or any other stress or in cases of cholecystoses. * Common in ICU patients. ACUTE CHOLECYSTITIS * Commonly it occurs in a patient with pre-existing chronic cholecystitis but often also can occur as a first presentation. * Usual cause is impacted gallstone in the Hamann's pouch, obstructing cystic duct. ref : SRB&;s manual of surgery,3 rd ed,pg no 576