**Core Concept**
Choledochal cysts are congenital anomalies of the extrahepatic bile duct characterized by dilatation of the bile ducts. They are classified into five types based on the location and extent of the dilatation. Accurate classification is crucial for determining the appropriate surgical approach and treatment.

**Why the Correct Answer is Right**
Type II choledochal cysts, also known as saccular diverticulum, are characterized by a localized dilatation of the extrahepatic bile duct. This type of cyst typically occurs as a diverticulum of the extrahepatic bile duct and is often associated with a normal or slightly dilated common bile duct. The saccular shape of this cyst is a key distinguishing feature from other types of choledochal cysts. The classification of choledochal cysts is based on the Todani modification of the Alonso-Lej classification, which includes Type I (fusiform dilatation), Type II (saccular diverticulum), Type III (cervical dilatation), Type IV (multiple dilatations), and Type V (carbamate diverticulum).

**Why Each Wrong Option is Incorrect**
**Option A:** Type I choledochal cysts are characterized by a fusiform dilatation of the extrahepatic bile duct, which is distinct from the saccular shape of Type II cysts.
**Option C:** Type III choledochal cysts involve a cervical dilatation of the common bile duct, which is not the same as a saccular diverticulum.
**Option D:** Type IV choledochal cysts are characterized by multiple dilatations of the bile duct, which is not a single saccular diverticulum.

**Clinical Pearl / High-Yield Fact**
When diagnosing choledochal cysts, it is essential to consider the Todani classification system to determine the appropriate surgical approach. Type II choledochal cysts often require a more complex surgical procedure due to their saccular shape and potential for complications.

**✓ Correct Answer: B. Type II**