**Core Concept**
Acute pancreatitis is a sudden inflammation of the pancreas, which can be caused by gallstones, excessive alcohol consumption, or other factors. The investigation of choice for diagnosing acute pancreatitis should be able to quickly identify the severity of the condition and guide further management.

**Why the Correct Answer is Right**
The investigation of choice for acute pancreatitis is serum lipase. Lipase is an enzyme produced by the pancreas that breaks down fats in the digestive system. In acute pancreatitis, lipase levels are significantly elevated, making it a sensitive and specific marker for diagnosing the condition. Serum lipase levels can also help determine the severity of acute pancreatitis, as higher levels are associated with more severe disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum amylase is another enzyme produced by the pancreas, but it is not as sensitive or specific as lipase for diagnosing acute pancreatitis. Amylase levels can also be elevated in other conditions, such as salivary gland disorders.
**Option B:** CT scan is a useful imaging modality for diagnosing acute pancreatitis, especially in severe cases, but it is not the investigation of choice for initial diagnosis.
**Option C:** Ultrasound is not typically used for diagnosing acute pancreatitis, although it may be used to evaluate for gallstones, a common cause of the condition.

**Clinical Pearl / High-Yield Fact**
The Ranson's criteria and APACHE II scoring system are useful for predicting the severity of acute pancreatitis, but serum lipase levels remain the most important initial diagnostic tool.

**Correct Answer: C. CT scan is a useful imaging modality for diagnosing acute pancreatitis, especially in severe cases, but it is not the investigation of choice for initial diagnosis.