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**
The question tests knowledge of the anatomy related to subarachnoid hemorrhage and the likely location of an aneurysm based on the site of bleeding. Subarachnoid hemorrhage often results from the rupture of an aneurysm, and the location of the hemorrhage can indicate the site of the aneurysm.
## **Why the Correct Answer is Right**
Blood isolated in the fourth ventricle on a CT scan suggests that the hemorrhage is located in the posterior fossa. The fourth ventricle is closely related to the **vertebrobasilar circulation**. Aneurysms in this circulation, particularly at the **junction of the posterior inferior cerebellar artery (PICA) and the vertebral artery** or at the **basilar tip**, can rupture and lead to hemorrhage in this area. The proximity of these aneurysms to the fourth ventricle makes them likely culprits for bleeding isolated to this structure.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not specify a location that would typically cause isolated bleeding into the fourth ventricle. Without specific anatomical correlation, it's hard to assess its likelihood directly, but given the context, it's less likely related to fourth ventricle bleeding.
- **Option B:** This option is incorrect because it implies a different vascular territory that might not directly explain bleeding isolated to the fourth ventricle.
- **Option C:** This option is incorrect because, similar to option A, without specific anatomical correlation to the fourth ventricle or posterior fossa, it's less directly related.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the location of subarachnoid hemorrhage on imaging can help identify the likely source of the bleeding. For instance, **aneurysms of the posterior communicating artery** often present with hemorrhage in the anterior part of the circle of Willis, while **basilar tip aneurysms** are associated with posterior circulation hemorrhages, potentially affecting the fourth ventricle.
## **Correct Answer:** . **Posterior Inferior Cerebellar Artery (PICA) or Vertebral Artery junction**