**Core Concept**: Cholangiocarcinoma is a malignant tumor originating from the cells of the bile ducts. Bismuth classification is a method used to categorize cholangiocarcinomas based on their location and involvement of the bile ducts. There are four types: type I, II, III, and IV.

**Why the Correct Answer is Right**: Type IV cholangiocarcinoma is a tumor that involves the hilar (Kuhn's) bile ducts and extends to involve the right and left hepatic ducts, but not the cystic duct. This type is characterized by infiltration of the hilar bile ducts and extension into the right and left hepatic ducts, but not involving the cystic duct.

**Why Each Wrong Option is Incorrect**:

A. Type I cholangiocarcinoma involves the cystic duct and/or the common hepatic duct but does not involve the hilar bile ducts.
B. Type II cholangiocarcinoma involves the left hepatic duct but does not involve the hilar bile ducts.
C. Type III cholangiocarcinoma involves the hilar bile ducts but does not extend into the right or left hepatic ducts.

**Clinical Pearl**: The Bismuth classification helps in determining the surgical approach for treatment of cholangiocarcinoma. Understanding the type helps in planning resections and determining the prognosis.

**Correct Answer:** B. Type II cholangiocarcinoma involves the left hepatic duct but does not involve the hilar bile ducts.