**Core Concept**
Spontaneous sub-arachnoid haemorrhage (SAH) is most commonly caused by the rupture of a cerebral aneurysm, particularly a berry aneurysm, which weakens the arterial wall and leads to bleeding into the subarachnoid space.

**Why the Correct Answer is Right**
Berry aneurysms are small, sac-like dilations of cerebral arteries, most commonly arising from the anterior communicating artery or branches of the internal carotid artery. These aneurysms are prone to rupture due to wall thinning and increased pressure, leading to sudden SAH. The condition is often associated with a sudden, severe headache ("worst headache of life") and is the most frequent cause of spontaneous SAH in adults. The pathophysiology involves arterial wall degeneration and rupture, not systemic conditions like hypertension or trauma.

**Why Each Wrong Option is Incorrect**
Option A: Trauma is a cause of sub-arachnoid haemorrhage, but it is not the *most common* cause of *spontaneous* SAH. Spontaneous SAH implies no external injury.
Option B: Hypertension contributes to vascular damage and can be a risk factor, but it is not the primary cause of SAH. It is more commonly linked to hypertensive encephalopathy or stroke, not spontaneous rupture.
Option D: Raised intracranial tension is a consequence of brain swelling or mass effect, not a direct cause of SAH. It may occur post-hemorrhage but does not initiate it.

**Clinical Pearl / High-Yield Fact**
Berry aneurysm rupture accounts for >80% of all spontaneous SAH cases. A classic sign is a thunderclap headache, and imaging (CT angiography or cerebral angiography) is essential for diagnosis and management. Early detection and surgical intervention are critical to prevent re-bleeding and death.

✓ Correct Answer: C. Berry aneurysm rupture