**Core Concept**
Serum amylase is an enzyme that plays a crucial role in the digestion of carbohydrates. Elevated levels of serum amylase can indicate pancreatitis, pancreatic trauma, or other conditions affecting the pancreas.

**Why the Correct Answer is Right**
The correct answer is related to the fact that serum amylase levels can be influenced by various factors, including the stage of digestion, the presence of pancreatic disease, and the use of certain medications. **Salivary amylase**, which is produced in the salivary glands, can also contribute to the total serum amylase level. However, in certain conditions, serum amylase may not be elevated due to the compensatory increase in salivary amylase production.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because serum amylase is typically elevated in acute pancreatitis, which is a common condition affecting the pancreas.

**Option B:** This option is incorrect because serum amylase can be elevated in chronic pancreatitis, which is a long-term inflammatory condition of the pancreas.

**Option C:** This option is incorrect because serum amylase is not typically elevated in pancreatic cancer, which is a type of cancer that affects the pancreas.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that serum amylase levels can be influenced by the stage of digestion, making it crucial to measure the enzyme at the correct time (usually between 12-24 hours after symptom onset) to accurately diagnose pancreatitis.

**Correct Answer: D. Pancreatic cancer**