Okay, let's tackle this question. The patient has epigastric pain radiating to the back, which is classic for pancreatitis. The serum amylase is normal, but USG shows gallstones and an enlarged pancreas. CT confirms the diagnosis.

First, the core concept here is acute pancreatitis. The main causes are gallstones and alcohol. Since the patient has gallstones, that's a key point. Amylase can be normal in some cases, but lipase is more specific. The CT is the gold standard for imaging in pancreatitis, so that's why it clinched the diagnosis.

The correct answer is acute pancreatitis. Why? Because the symptoms and findings fit. Gallstones can obstruct the pancreatic duct, leading to inflammation. The enlarged pancreas on USG and CT confirmation support this.

Now, the wrong options. Let's see. If the options were other conditions like peptic ulcer, cholecystitis, or pancreatic cancer, those would be the distractors. Peptic ulcer pain doesn't radiate to the back as much. Cholecystitis is more right upper quadrant and might have elevated amylase but CT would show gallbladder issues. Pancreatic cancer could present similarly but typically has weight loss and jaundice.

Clinical pearl: Remember that amylase can be normal in pancreatitis; lipase is more reliable. Also, CT is the best imaging modality for acute pancreatitis. Gallstones are the most common cause in this scenario.

**Core Concept**
The scenario describes a classic presentation of **acute pancreatitis**, often caused by gallstones obstructing the pancreatic duct. Key diagnostic clues include epigastric pain radiating to the back, gallstones on imaging, and pancreatic enlargement. While serum amylase may be normal, **lipase** is more sensitive and specific for this condition. CT is the gold standard for assessing severity and complications.

**Why the Correct Answer is Right**
Acute pancreatitis occurs when gallstones block the ampulla of Vater, causing pancreatic enzyme activation and autodigestion. The enlarged pancreas on ultrasound and confirmation via CT (which visualizes edema, necrosis, or fluid collections) solidify the diagnosis. Even with normal amylase, CT’s ability to detect parenchymal changes makes it definitive.

**Why Each Wrong Option is Incorrect**
**Option A:** Peptic ulcer disease presents with epigastric pain but does not cause pancreatic enlargement or radiate to the back.
**Option B:** Acute cholecystitis causes right upper quadrant pain, Murphy’s sign, and gallbladder wall thickening—not pancreatic enlargement.
**Option C:** Pancreatic cancer typically presents with weight loss, jaundice, and a mass on imaging, not acute inflammatory changes.

**Clinical Pearl / High-Yield Fact**
**"Lipase > amylase"** – Always check serum lipase in suspected pancreatitis, as amylase can be normal or elevated due to other causes (e.g., salivary gland issues). CT is superior to ultrasound for staging pancreatitis severity. Gallstones are the **most common cause** in adults under 60.

**Correct Answer: C. Acute pancreatitis**

✓ Correct Answer: A. Acute pancreatitis