**Core Concept**
Acute pancreatitis is an inflammatory condition of the pancreas that can lead to various complications, including the formation of fluid collections. The pancreas is a vital organ responsible for producing digestive enzymes and hormones, and its inflammation can cause a cascade of events leading to tissue damage.

**Why the Correct Answer is Right**
The most common complication of acute pancreatitis is the formation of a pancreatic pseudocyst, which is a fluid collection within the pancreas or adjacent tissues. Pseudocysts are usually sterile and contain pancreatic juice, cellular debris, and necrotic tissue. They can cause abdominal pain, discomfort, and potentially life-threatening complications, such as rupture or infection. The formation of pseudocysts is thought to occur due to the breakdown of pancreatic tissue and the leakage of pancreatic juice into the surrounding tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** Pancreatic abscess is a collection of pus within the pancreas, typically occurring in cases of infected pancreatic necrosis. While it is a serious complication, it is not the most common one.
**Option C:** Pancreatic phlegmon refers to a localized inflammatory response within the pancreas, often seen in the early stages of acute pancreatitis. However, it is not a specific complication and does not typically form a fluid collection.
**Option D:** Pleural effusion is a complication of acute pancreatitis, but it is not as common as pseudocyst formation. Pleural effusion occurs when pancreatic enzymes leak into the pleural space, causing inflammation and fluid accumulation.

**Clinical Pearl / High-Yield Fact**
When managing a patient with acute pancreatitis, it is essential to monitor for signs of complications, such as abdominal pain, fever, and changes in vital signs. Early detection and intervention can prevent the progression to more severe complications, including pseudocyst formation.

**✓ Correct Answer: B. Pseudocyst**