**Core Concept**
Amylase is a pancreatic enzyme that can leak into the bloodstream during acute pancreatitis, causing elevated serum levels. The enzyme's half-life in the blood is approximately 6-8 hours, which can lead to normalization of serum amylase levels within 7 days if the pancreatitis is mild and resolves on its own.

**Why the Correct Answer is Right**
If serum amylase remains elevated after 7 days, it may indicate a more severe or complicated course of pancreatitis, such as pancreatic necrosis, walled-off necrosis, or secondary complications like infection or pseudocyst formation. The pancreas may also be producing more amylase than usual in response to ongoing inflammation or injury.

**Why Each Wrong Option is Incorrect**
**Option A:** Pancreatic cancer can indeed cause chronic elevation of serum amylase levels, but the question specifically asks for causes of elevated amylase after 7 days of acute pancreatitis.

**Option B:** Pancreatic pseudocyst formation is a possible complication of acute pancreatitis, and it can lead to persistent elevation of serum amylase levels. Therefore, this option is not correct.

**Option C:** Pancreatic necrosis is a severe complication of acute pancreatitis that can lead to persistent elevation of serum amylase levels. This option is a possible cause of elevated amylase after 7 days.

**Clinical Pearl / High-Yield Fact**
Remember that serum amylase levels can be elevated in other conditions, such as salivary gland disease or intestinal obstruction, so it's essential to consider the clinical context and imaging findings when interpreting amylase levels.

**Correct Answer: A. Pancreatic cancer.**