Acute cholangitis results from bacterial infection of the biliary tree, most commonly caused by obstruction from choledocholithiasis, strictures, or stent occlusion. Bile stasis proximal to the obstruction allows bacterial colonization (typically E. coli, Klebsiella, Enterococcus) and ascending infection. Increased intraluminal pressure forces bacteria and endotoxins into the hepatic sinusoids and systemic circulation, producing bacteremia and sepsis. The classic Charcot triad (fever, jaundice, RUQ pain) progresses to Reynolds pentad (adding altered mental status and hypotension) in severe cases.