List six causes of pneumobilia.

1. Iatrogenic following ERCP or sphincterotomy
2. Surgery (e.g., biliary enteric anastomosis)
3. Fistula formation between the biliary tract and the bowel, such as occurs with peptic ulcer erosion
4. Malignancy with erosion
5. Gallstone ileus (i.e., erosion of a gallstone into the bowel)
6. Infection by a gas-forming organism e.g., Clostridia, Bacteroides fragilis)