Okay, let's tackle this question. The patient has epigastric pain radiating to the back for three days, normal serum amylase, cholelithiasis on USG, and an enlarged pancreas on CT. The answer is probably pancreatitis, but wait, the amylase is normal. Hmm, I remember that in acute pancreatitis, amylase can be normal in some cases, especially if it's early or if there's another cause. The presence of cholelithiasis suggests gallstone pancreatitis. But what about the other options? Let's see.

The core concept here is acute pancreatitis and its differential diagnosis. Gallstones are a common cause of pancreatic duct obstruction. Amylase levels might not always be elevated, especially if the inflammation is not severe or if it's in the early stages. The CT scan would show pancreatic enlargement, which is a key sign.

Why is the correct answer right? Gallstone pancreatitis occurs when a stone blocks the pancreatic duct at the ampulla of Vater, leading to enzyme accumulation and autodigestion. The pain radiating to the back is classic. Even with normal amylase, CT and clinical context support the diagnosis.

Now, the wrong options. Let's say the options include other causes like peptic ulcer, cholecystitis, or pancreatic cancer. Peptic ulcer pain is more localized, doesn't radiate to the back as much. Cholecystitis would have right upper quadrant pain and Murphy's sign. Pancreatic cancer might have weight loss and jaundice. The key here is the combination of cholelithiasis and pancreatic enlargement.

Clinical pearl: Remember that normal amylase doesn't rule out pancreatitis. CT is the gold standard for confirmation when amylase/lipase is inconclusive. Gallstones are a leading cause, so cholelithiasis on imaging is a strong indicator.

**Core Concept**
This case examines *acute pancreatitis* secondary to *gallstone-induced obstruction* of the pancreatic duct. Key features include epigastric/back pain, cholelithiasis (gallstones), and pancreatic enlargement on imaging. Serum amylase may be normal in early or mild cases, underscoring the role of imaging (e.g., CT) in diagnosis.

**Why the Correct Answer is Right**
Gallstone pancreatitis occurs when a stone obstructs the **ampulla of Vater**, causing pancreatic enzyme accumulation and autodigestion. The **epigastric pain radiating to the back**, **normal serum amylase** (which can occur in early or mild disease), and **enlarged pancreas on CT** (showing edema or inflammation) confirm the diagnosis. Cholelithiasis is the most common cause of acute pancreatitis (40–70% of cases).

**Why Each Wrong Option is Incorrect**
**Option A:** *Peptic ulcer disease* presents with gnawing epigastric pain, often relieved by food/antacids, and lacks pancreatic enlargement.
**Option B:** *Acute cholecystitis* causes right upper quadrant pain, fever, and positive Murphy’s sign, without pancreatic involvement.
**Option C:** *Pancreatic cancer* typically presents with progressive jaundice, weight loss, and a mass, not

✓ Correct Answer: B. Acute Pancreatitis