**Question:** A patient with suspected subarachnoid haemorrhage presents with blood isolated in the fourth ventricle on a CT scan. Aneurysmal rupture is likely to have resulted from:

**Core Concept:** Subarachnoid haemorrhage (SAH) is a type of stroke caused by bleeding into the subarachnoid space, usually due to rupture of a cerebral aneurysm. The fourth ventricle is a small, U-shaped cavity located at the base of the brain, surrounded by the cerebellum, fourth ventricle fluid compartments, and the brainstem.

**Why the Correct Answer is Right:** When blood accumulates in the fourth ventricle, it suggests that the source of bleeding is from the cerebellomedullary cistern, which is a space between the cerebellum and the brainstem. The fourth ventricle is located close to the region where a ruptured aneurysm can cause bleeding. In such cases, the bleeding originates from a ruptured aneurysm located at the base of the brain, near the fourth ventricle.

**Why Each Wrong Option is Incorrect:**

A. **Option A (third ventricle):** The third ventricle is located between the cerebral hemispheres, not the base of the brain where the fourth ventricle is situated. Hence, bleeding in the third ventricle is less likely in this scenario.

B. **Option B (intracranial space):** While bleeding into the intracranial space is a possibility, the question specifies that the blood is isolated in the fourth ventricle, making this option less likely.

C. **Option C (cerebellum):** The cerebellum is a part of the brain, but the presence of blood in the fourth ventricle indicates bleeding from a different region.

D. **Option D (cerebral cortex):** The cerebral cortex is not the source of bleeding, as the question states that the blood is isolated in the fourth ventricle. The bleeding is more likely from a source in the base of the brain, which is the fourth ventricle in this case.

**Clinical Pearl:** When evaluating a patient with suspected SAH, focusing on the location of bleeding helps narrow down the potential cause. In this case, the isolated blood in the fourth ventricle suggests a ruptured aneurysm at the base of the brain, which is more likely than bleeding from other brain regions like the cerebral cortex, third ventricle, or cerebellum.

**Correct Answer:** Option D (cerebral cortex) is the correct answer because it is the most likely source of bleeding given the isolated blood in the fourth ventricle. A ruptured aneurysm at the base of the brain is the primary cause of SAH, and bleeding into the cerebral cortex would also result in diffuse SAH, not localized bleeding in a specific ventricle.