A patient of post-cholecystectomy biliary stricture has undergone an ERCP three days ago. Following this she has developed acute cholangitis. The most likely organism is-
**Core Concept**
The question is testing the understanding of the typical microbiology of acute cholangitis, specifically in the context of a biliary stricture and recent ERCP. Acute cholangitis is a bacterial infection of the biliary tract, often caused by the ascent of bacteria from the duodenum into the biliary tree.
**Why the Correct Answer is Right**
In the setting of a biliary stricture and recent ERCP, the most likely organism is *Escherichia coli* (E. coli). This is because E. coli is a common commensal organism of the gastrointestinal tract, and its presence in the biliary tree can lead to infection. The biliary stricture provides a conduit for the bacteria to ascend into the biliary tree, and the recent ERCP may have introduced bacteria into the biliary system. E. coli is a gram-negative bacterium that can produce virulence factors such as lipopolysaccharide, which contributes to its pathogenicity.
**Why Each Wrong Option is Incorrect**
* **Option A:** Klebsiella pneumoniae is another gram-negative bacterium that can cause cholangitis, but it is less common than E. coli in this setting.
* **Option B:** Staphylococcus aureus is a gram-positive bacterium that is more commonly associated with skin and soft tissue infections, rather than biliary tract infections.
* **Option D:** Pseudomonas aeruginosa is a gram-negative bacterium that can cause a variety of infections, but it is not the most likely organism to cause acute cholangitis in this setting.
**Clinical Pearl / High-Yield Fact**
When managing a patient with acute cholangitis, it is essential to consider the microbiology of the infection and select antibiotics that are effective against the likely causative organisms. In this case, a combination of antibiotics that cover E. coli, such as a third-generation cephalosporin or a fluoroquinolone, may be necessary.
**Correct Answer: C. Escherichia coli**