## **Core Concept**
The superior haemorrhoidal veins are part of the venous drainage system of the anal canal. They are involved in draining the upper part of the anal canal. Understanding their drainage pathway and characteristics is crucial for managing conditions like hemorrhoids.

## **Why the Correct Answer is Right**
The correct answer can be deduced by understanding the anatomy and physiology of the anal canal's venous drainage. The superior haemorrhoidal veins drain into the **inferior mesenteric vein**, which is a key part of the portal venous system. This is accurate because the superior haemorrhoidal veins are responsible for draining the upper part of the anal canal, and this area is drained into the systemic circulation via the portal system.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option might seem correct but let's evaluate all options.
- **Option B:** The statement that the superior haemorrhoidal veins **have no valves** is actually true and is a characteristic feature of veins in general, but particularly relevant here as it influences the direction of blood flow and pressure build-up.
- **Option C:** The superior haemorrhoidal veins **leave the anal canal at the pectinate line**. This is correct anatomically as the pectinate line (or dentate line) marks the boundary between the upper and lower parts of the anal canal, and the superior haemorrhoidal veins drain the area above this line.
- **Option D:** The statement that they **cause external haemorrhoids** is incorrect. External hemorrhoids are caused by the occlusion of the inferior haemorrhoidal veins, not the superior haemorrhoidal veins.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **hemorrhoids can be classified based on their location relative to the pectinate line** into internal (above the line, drained by superior haemorrhoidal veins) and external (below the line, drained by inferior haemorrhoidal veins). This classification is crucial for determining the treatment approach.

## **Correct Answer: B.**