**Question:** The first investigation of choice in a patient with suspected subarachnoid haemorrhage should be:

A. Computerized Tomography (CT) scan
B. Cerebrospinal fluid (CSF) analysis
C. Magnetic Resonance Imaging (MRI)
D. Digital Subtraction Angiography (DSA)

**Core Concept:** Subarachnoid haemorrhage is a life-threatening condition resulting from bleeding into the subarachnoid space, usually due to a ruptured cerebral aneurysm. The primary goal of initial diagnostic investigation is to confirm the presence and identify the source of the bleeding, allowing for timely and appropriate management.

**Why the Correct Answer is Right:** DSA (Digital Subtraction Angiography) is the gold standard investigation for imaging the vasculature and identifying the source of the bleeding, which is crucial in such cases. DSA provides detailed angiographic images of cerebral vessels and allows visualization of aneurysms, arteriovenous malformations, and other vascular abnormalities. It is the most sensitive and specific imaging modality for detecting cerebral aneurysms causing subarachnoid haemorrhage.

**Why Each Wrong Option is Incorrect:**

A. CT (Computerized Tomography) scan: While CT scan can detect the presence of blood in the subarachnoid space, it lacks the ability to visualize the vasculature and identify the source of the bleeding. This is crucial information for adequate management.

B. CSF (Cerebrospinal Fluid) analysis: CSF analysis is not recommended as a first-line investigation for subarachnoid haemorrhage. Analyzing CSF may reveal signs of aneurysmal rupture, but it lacks the ability to visualize the vasculature and identify the source of the bleeding.

C. MRI (Magnetic Resonance Imaging): MRI can detect blood in the subarachnoid space, but it is less sensitive and specific for detecting aneurysmal rupture compared to DSA. MRI is not as accurate as DSA in identifying the source of bleeding and the underlying cause.

**Clinical Pearl:** Early identification of the source of bleeding and the underlying cause is crucial for the management of subarachnoid haemorrhage. DSA provides the necessary information for prompt and appropriate intervention.