**Core Concept**
Choledochal cysts are congenital anomalies of the bile ducts, characterized by dilatation of the intrahepatic or extrahepatic bile ducts. These cysts can be classified into several types based on their location and morphology.

**Why the Correct Answer is Right**
The most common type of choledochal cyst is Type I, also known as a fusiform or saccular cyst. This type accounts for approximately 90% of all choledochal cysts. Type I cysts are characterized by a dilated segment of the common bile duct, often involving the entire length of the duct. The pathophysiology of choledochal cysts is not fully understood, but it is thought to involve a combination of genetic and environmental factors that lead to abnormal development of the bile ducts.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as Type II choledochal cysts, also known as a diverticulum, are less common and account for only a small percentage of cases.
**Option B:** This option is incorrect as Type III choledochal cysts, also known as a choledochocele, are a rare type of cyst that involves the distal common bile duct.
**Option C:** This option is incorrect as Type IV choledochal cysts involve multiple dilated bile ducts, but are not the most common type.

**Clinical Pearl / High-Yield Fact**
Choledochal cysts are often associated with an increased risk of cholangiocarcinoma, a type of bile duct cancer. Patients with a history of choledochal cysts should be closely monitored for signs of malignancy.

**Correct Answer: C. Type I**