## **Core Concept**
The patient's presentation of epigastric pain radiating to the back, along with an enlarged pancreas and gallbladder stones, suggests a pancreatic pathology. The normal serum amylase levels do not rule out pancreatitis, as levels can return to normal within 24-48 hours after the onset of symptoms.

## **Why the Correct Answer is Right**
The clinical presentation and imaging findings are suggestive of **acute pancreatitis**. Acute pancreatitis can present with epigastric pain radiating to the back and is often associated with gallstones. The USG finding of gallbladder stones and an enlarged pancreas supports this diagnosis. A CT scan can clinch the diagnosis by showing pancreatic enlargement, necrosis, or inflammation.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is not provided, but typically, incorrect options might include conditions like cholecystitis, which can cause epigastric pain but usually doesn't cause pancreatic enlargement.
- **Option B:** Similarly, this option is not provided, but another incorrect option could be a gastrointestinal infection or gastritis, which wouldn't explain the pancreatic enlargement.
- **Option D:** Without the specific details of option D, it's hard to address directly, but any option not suggesting pancreatitis would be incorrect given the clinical scenario.

## **Clinical Pearl / High-Yield Fact**
In acute pancreatitis, serum amylase levels may not always be elevated, especially if the patient presents late in the course of the disease. Imaging studies like USG and CT are crucial for diagnosis. A key point to remember is that gallstones are a common cause of acute pancreatitis, and their presence along with clinical symptoms should prompt further investigation.

## **Correct Answer:** C.