**Core Concept**
Pseudocyst of the pancreas is a fluid collection that forms in the abdomen due to pancreatic inflammation or trauma. It is a common complication of pancreatitis and can cause significant morbidity if not managed properly.

**Why the Correct Answer is Right**
The commonest complication of pseudocyst of the pancreas is rupture. Rupture of a pancreatic pseudocyst can lead to severe abdominal pain, bleeding, and potentially life-threatening complications. This is often due to the high pressure within the pseudocyst causing it to break through the surrounding tissues and structures, such as the stomach, small intestine, or major blood vessels. The rupture can also lead to infection of the pseudocyst, known as pancreatic abscess.

**Why Each Wrong Option is Incorrect**
**Option A:** Infection of the pseudocyst is a potential complication, but it is not the most common one. Infection occurs when the pseudocyst becomes contaminated with bacteria, leading to abscess formation.
**Option B:** Haemorrhage from a pseudocyst is a rare complication, often due to erosion into a major blood vessel.
**Option C:** Malignant transformation of a pseudocyst into a true cystic neoplasm is a rare occurrence.

**Clinical Pearl / High-Yield Fact**
The presence of a pseudocyst in a patient with pancreatitis should prompt careful monitoring for signs of rupture, such as severe abdominal pain, tachycardia, and hypotension.

**Correct Answer:** D. Rupture