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: Medicine
Gas in the biliary tree on plain abdominal X-ray is suggestive of: 
Select and explain. 
A. Empyema of the gallbladder, B. Mirizzi syndrome, C. Biliary enteric fistula, D. Porcelain gallbladder
C: Biliary enteric fistula
Answer: C: Biliary enteric fistula 
Explanation: Clinically "silent" biliary-enteric fistulas occurring as a complication of acute cholecystitis and it can be diagnosed by finding gas in the biliary tree on plain abdominal films. Barium contrast studies or endoscopy of the upper gastrointestinal tract or colon may demonstrate the fistula. Reference: Harrisons Principles of Internal Medicine, 18th Edition, Page 2622