**Question:** 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

A. Escherichia coli
B. Klebsiella pneumoniae
C. Pseudomonas aeruginosa
D. Enterococcus faecalis

**Core Concept:**

Post-cholecystectomy biliary stricture is a complication that can occur after cholecystectomy (gallbladder surgery), leading to a narrowing of the bile ducts. Endoscopic retrograde cholangiopancreatography (ERCP) is a procedure used to visualize the biliary tree and pancreatic duct for diagnosis and treatment. Acute cholangitis is an acute inflammation of the bile ducts, often caused by bacterial infection.

**Why the Correct Answer is Right:**

The correct answer is **Klebsiella pneumoniae**. Klebsiella pneumoniae is a gram-negative bacillus that is commonly found in the gastrointestinal tract and respiratory tract. It is known for causing severe infections, including cholangitis. In this case, the post-ERCP procedure might have led to bile duct disruption or injury, providing a portal for Klebsiella pneumoniae to enter and cause an infection.

**Why Each Wrong Option is Incorrect:**

A. **Escherichia coli (E. coli)** is another gram-negative bacillus, but it is generally less virulent than Klebsiella pneumoniae and less commonly associated with cholangitis.

B. **Klebsiella pneumoniae (Option C)** is the correct answer, as mentioned above.

C. **Pseudomonas aeruginosa** is a different gram-negative bacillus, typically associated with hospital-acquired infections and less commonly involved in post-ERCP cholangitis.

D. **Enterococcus faecalis** is a gram-positive bacterium, not relevant to the gram-negative bacilli typically causing cholangitis.

**Clinical Pearls:**

1. Post-ERCP cholangitis is an important clinical scenario to understand, as it highlights the potential complications and appropriate antibiotic therapy in such cases.
2. Rapid antibiotic therapy targeting gram-negative bacteria, such as Klebsiella pneumoniae, is essential for managing post-ERCP cholangitis.