**Core Concept**
Internal haemorrhoids are a type of haemorrhoid that originates above the dentate line, within the anal canal's mucosal lining. They are typically treated with rubber band ligation, sclerotherapy, or other minimally invasive procedures. This is because internal haemorrhoids are not exposed to the external environment, making them more amenable to conservative treatments.

**Why the Correct Answer is Right**
Internal haemorrhoids are ideal for sclerotherapy because the sclerosing agent can be injected directly into the haemorrhoidal tissue, causing fibrosis and shrinkage of the haemorrhoid. This is particularly effective for first- and second-degree internal haemorrhoids. The injection of a sclerosing agent, such as phenol or quinine, induces inflammation and scarring, which ultimately leads to the resolution of the haemorrhoid. This procedure is often performed under local anaesthesia and is a common treatment option for internal haemorrhoids.

**Why Each Wrong Option is Incorrect**
**Option A:** External haemorrhoids are typically located below the dentate line and are not suitable for sclerotherapy, as the sclerosing agent may cause significant pain and discomfort.
**Option C:** Prolapsed haemorrhoids may require more aggressive treatment, such as rubber band ligation or surgical excision, due to their more severe nature.
**Option D:** Thrombosed haemorrhoids are often treated with surgical excision or thrombectomy, as the thrombosis can cause significant pain and discomfort.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the treatment of haemorrhoids often depends on the patient's symptoms, the severity of the haemorrhoids, and the presence of any underlying medical conditions. A thorough physical examination and medical history are crucial in determining the most appropriate treatment option.

**✓ Correct Answer: B. Internal haemorrhoids**