**Core Concept**
ERCP (Endoscopic Retrograde Cholangiopancreatography) is a procedure used to visualize and treat biliary and pancreatic ducts. The most common post-procedural complication arises due to the stimulation of pancreatic secretions, which can trigger acute pancreatitis, especially when contrast is injected or during cannulation of the common bile duct.

**Why the Correct Answer is Right**
Acute pancreatitis is the most frequent complication after ERCP, occurring in approximately 1–3% of cases. It results from the backflow of pancreatic enzymes into the ductal system during catheterization or contrast injection, leading to pancreatic acinar cell activation and inflammation. The risk is higher with prolonged cannulation, use of high-osmolarity contrast, or pre-existing pancreatic disease. This complication is well-documented in clinical literature and is the leading post-ERCP adverse event.

**Why Each Wrong Option is Incorrect**
Option B: Acute cholangitis is a rare complication, typically occurring due to biliary obstruction or infection, not the most common post-ERCP event.
Option C: Acute cholecystitis is not directly related to ERCP and is more commonly associated with gallstone disease or gallbladder pathology.
Option D: Duodenal perforation is a very rare, life-threatening complication, occurring in less than 0.1% of cases and is not the most common.

**Clinical Pearl / High-Yield Fact**
Always consider acute pancreatitis as the top post-ERCP complication—patients should be monitored for symptoms like severe abdominal pain, nausea, and elevated amylase levels within 24–72 hours. Prevention includes using low-osmolarity contrast and minimizing catheter dwell time.

✓ Correct Answer: A. Acute Pancreatitis