## **Core Concept**
The **Valves of Houston**, also known as the spiral valves or valves of Heister, are found in the cystic duct. These valves play a role in regulating bile flow from the gallbladder into the common hepatic duct and then into the common bile duct.

## **Why the Correct Answer is Right**
The Valves of Houston are small, spiral mucosal folds within the cystic duct. They are crucial for maintaining a continuous flow of bile and preventing the cystic duct from becoming obstructed by sludge or small stones. The presence of these valves ensures that the cystic duct remains patent, facilitating the flow of bile into and out of the gallbladder.

## **Why Each Wrong Option is Incorrect**
- **Option A:** [Insert explanation for option A, e.g., This option is incorrect because...]
- **Option B:** [Insert explanation for option B, e.g., This option is incorrect because...]
- **Option C:** [Insert explanation for option C, e.g., This option is incorrect because...]

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the Valves of Houston help prevent the collapse of the cystic duct, ensuring unobstructed flow of bile. This is particularly relevant during surgical procedures like cholecystectomy, where understanding the anatomy of the cystic duct and its valves can be critical.

## **Correct Answer:** D.