## Core Concept
Biliary obstruction occurs when the flow of bile from the liver and gallbladder into the small intestine is blocked. This obstruction can occur at various levels, including the intrahepatic bile ducts, the common hepatic duct, the common bile duct, and the ampulla of Vater. Causes of biliary obstruction can be broadly categorized into intrinsic (within the bile duct) and extrinsic (outside the bile duct) factors.

## Why the Correct Answer is Right
Gallstones are a common cause of biliary obstruction. They can obstruct the bile ducts at various levels, most commonly at the cystic duct or the common bile duct. This obstruction can lead to jaundice, cholangitis, and pancreatitis. The mechanism involves physical blockage of the bile duct by the stone, preventing the flow of bile into the intestine.

## Why Each Wrong Option is Incorrect
* **Option A:** Pancreatic carcinoma can cause biliary obstruction by compressing or invading the common bile duct. This is a classic cause of obstructive jaundice and is often associated with a palpable gallbladder (Courvoisier's sign).
* **Option B:** Cholangiocarcinoma, a cancer of the bile duct, directly causes biliary obstruction by obstructing the bile ducts at various levels. It can present with obstructive jaundice and may be associated with a stricture or mass in the bile duct.
* **Option D:** Mirizzi syndrome, a condition where a gallstone becomes impacted in the cystic duct or the Hartmann's pouch of the gallbladder and causes obstruction of the common hepatic duct or common bile duct, is another cause of biliary obstruction.

## Clinical Pearl / High-Yield Fact
A key clinical point to remember is that while gallstones are a very common cause of biliary obstruction, not all gallstones cause obstruction. The presence of gallstones does not necessarily equate to biliary obstruction; it depends on the location and size of the stones.

## Correct Answer: B.