This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Pancreas, Subject name: Surgery
A 60-year-old diabetic man is admitted to the hospital with a diagnosis of acute cholecystitis. The WBC count is 28,000, and a plain film of the abdomen and CT scan show evidence of intramural gas in the gallbladder. What is the most likely diagnosis? 
Select and explain. 
A. Emphysematous gallbladder, B. Acalculous cholecystitis, C. Cholangiohepatitis, D. Sclerosing cholangitis
A: Emphysematous gallbladder
Answer: A: Emphysematous gallbladder 
Explanation: Emphysematous cholecystitis is caused by gas-forming organisms. On a plain x-ray of the abdomen, gas may be seen within the wall of the gallbladder. Clinically, the patient has rapidly progressive sepsis, RUQ pain, fever, and hemodynamic instability. The disease primarily affects diabetic men. Treatment with laparotomy and cholecystectomy is urgent to avoid complications. Air within the biliary tree (not gallbladder wall) may be seen in gallstone ileus, after biliary- enteric anastomosis or after sphincterotomy.