**Question:** Primary 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. Angiography

**Correct Answer:** A. Computerized Tomography (CT) scan

**Core Concept:** Subarachnoid haemorrhage (SAH) is a life-threatening condition that occurs when a weakened blood vessel in the brain ruptures, leading to bleeding between the brain and the arachnoid mater. Early diagnosis and intervention are crucial for improving patient outcomes.

**Why the Correct Answer is Right:** CT scan is the primary investigation of choice for suspected SAH due to its high sensitivity and specificity in detecting blood in the subarachnoid space. The characteristic finding of SAH on CT scan is the "Wispy Sign", which is a diffuse, symmetric, and punctate pattern of hyperdensity in the subarachnoid space.

**Why Each Wrong Option is Incorrect:**

B. Cerebrospinal Fluid (CSF) analysis: Although CSF analysis can detect blood in the subarachnoid space, it has low sensitivity and specificity in detecting SAH, making it an unsuitable first-line investigation.

C. Magnetic Resonance Imaging (MRI): MRI is a valuable tool for detecting SAH, but its sensitivity is lower than that of CT scan due to the presence of artefacts caused by blood products in the subarachnoid space.

D. Angiography: Angiography is an invasive procedure involving the injection of contrast medium into the blood vessels, which can identify the source of the bleeding. However, it is not the primary investigation for SAH diagnosis. CT scan serves as the initial investigation to confirm the presence of SAH and guide further management.

**Clinical Pearls:**

1. In suspected cases of SAH, the initial diagnostic tool should be a reliable, non-invasive, and readily available method. CT scan meets these criteria, making it the preferred option.
2. MRI is valuable for follow-up imaging after CT scan or for monitoring the progression of the disease, but is not the first choice for diagnosis.
3. Angiography is essential for identifying the source of the bleeding, but is not a primary diagnostic tool and should be performed after the initial CT scan.
4. CSF analysis is a valuable tool for identifying the etiology of SAH, such as identifying the presence of blood in the cerebrospinal fluid. However, it is not the primary diagnostic tool due to low sensitivity and specificity.