## **Core Concept**
The investigation of choice for subarachnoid hemorrhage (SAH) involves imaging techniques that can accurately visualize the bleeding in the subarachnoid space. SAH is a life-threatening condition that requires immediate diagnosis and treatment. The most commonly used imaging modalities for SAH include Computed Tomography (CT) scan, Magnetic Resonance Imaging (MRI), and Lumbar Puncture (LP).

## **Why the Correct Answer is Right**
The correct answer, **CT Head**, is the investigation of choice for SAH because it is quick, widely available, and highly sensitive for detecting acute blood in the subarachnoid space. CT scans can show the location and extent of the hemorrhage and are particularly useful in the acute setting. Non-contrast CT head is typically the first imaging done because it quickly identifies most cases of SAH by showing blood in the basal cisterns or ventricles.

## **Why Each Wrong Option is Incorrect**
- **Option A:** MRI, while useful for detecting SAH, especially in the subacute or chronic phase, is not the first choice in the acute setting due to its longer scanning time, lesser availability, and issues with monitoring acutely ill patients.
- **Option B:** Lumbar Puncture (LP) can be used to diagnose SAH, especially if the CT is negative but clinical suspicion remains high. However, LP is invasive and may not be suitable as a first-line investigation due to the risk of precipitating brain herniation in cases of raised intracranial pressure.
- **Option D:** Ultrasound is not typically used for diagnosing SAH as it cannot penetrate the skull and thus cannot image the brain.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in cases of suspected SAH with a negative CT head, a lumbar puncture should be performed to check for blood or bilirubin in the cerebrospinal fluid, which can indicate SAH. Also, remember that a CT scan is most sensitive when done within the first 24-48 hours after the hemorrhage.

## **Correct Answer:** C. CT Head.