**Core Concept**
ERCP (Endoscopic Retrograde Cholangiopancreatography) is a diagnostic and therapeutic procedure used to visualize the biliary and pancreatic ductal systems. In the context of pancreatitis, ERCP is performed to identify or treat complications related to the biliary or pancreatic ducts.

**Why the Correct Answer is Right**
ERCP is particularly useful in pancreatitis to diagnose or manage associated cholangitis, which is an infection of the bile duct. This is often due to gallstones obstructing the common bile duct, leading to a buildup of bacteria and pus in the biliary system. ERCP allows for the direct visualization of the bile ducts and the removal of obstructing gallstones or other debris. This can help to prevent further complications, such as sepsis or abscess formation.

**Why Each Wrong Option is Incorrect**
**Option A:** Gallstones are a common cause of pancreatitis, but ERCP is not primarily performed to diagnose gallstones (ultrasound or CT scan are more commonly used for this purpose).

**Option C:** Ascites, or fluid accumulation in the peritoneal cavity, is not a direct indication for ERCP in pancreatitis. While ascites can be a complication of pancreatitis, ERCP is not the primary diagnostic tool for this condition.

**Option D:** Pancreatic divisum is a congenital anomaly of the pancreatic ductal system, but it is not a common indication for ERCP in the setting of acute pancreatitis.

**Option E:** Annular pancreas is a rare congenital anomaly where pancreatic tissue surrounds the second part of the duodenum, but it is not a primary indication for ERCP in pancreatitis.

**Clinical Pearl / High-Yield Fact**
ERCP is a valuable diagnostic and therapeutic tool in the management of pancreatitis, particularly when there is suspicion of an associated biliary tract complication. However, it should be performed by experienced endoscopists in a controlled environment, as it carries a higher risk of complications compared to other imaging modalities.

**Correct Answer:** B. Associated cholangitis.