**Core Concept**
The triad of diabetes, gallstones, and steatorrhoea is a classic presentation of a rare pancreatic disorder. This condition is characterized by the destruction of pancreatic islet cells, leading to diabetes, and the impairment of pancreatic exocrine function, resulting in malabsorption and steatorrhoea.

**Why the Correct Answer is Right**
The correct answer is associated with a type of tumour that arises from the pancreas and causes the destruction of both the endocrine and exocrine pancreatic tissues. The tumour is a **Pancreatic Neuroendocrine Tumour (PNET)**, specifically a **VIPoma**. VIPomas are rare, aggressive tumours that produce excessive amounts of vasoactive intestinal peptide (VIP), which leads to the characteristic symptoms of severe diarrhea (steatorrhoea), hypokalemia, and sometimes diabetes. The destruction of pancreatic islet cells results in insulin deficiency and hyperglycemia, while the impairment of pancreatic exocrine function leads to malabsorption and steatorrhoea.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because **Pseudopancreatic Tumours** are not associated with the triad of diabetes, gallstones, and steatorrhoea. Pseudopancreatic tumours are actually a type of pancreatitis that mimics a pancreatic tumour on imaging studies.
* **Option B:** This option is incorrect because **Pancreatic Adenocarcinoma** is a type of pancreatic cancer that is often associated with jaundice and weight loss, but not typically with the triad of diabetes, gallstones, and steatorrhoea.
* **Option C:** This option is incorrect because **Pancreatic Pseudocysts** are fluid collections that can occur in the pancreas as a result of pancreatitis or pancreatic trauma, but they are not associated with the triad of diabetes, gallstones, and steatorrhoea.

**Clinical Pearl / High-Yield Fact**
VIPomas are often referred to as "pancreatic endocrine tumours" and can be difficult to diagnose due to their rarity and non-specific symptoms. A high index of suspicion and appropriate diagnostic testing, including imaging studies and biochemical assays, are essential for making an early diagnosis.

**Correct Answer: D. VIPoma**