## Core Concept
The core concept here revolves around the diagnosis of subarachnoid hemorrhage (SAH), which is a life-threatening condition often presenting with sudden onset headache and altered sensorium. SAH is typically due to the rupture of an aneurysm or less commonly due to trauma. The diagnostic investigation of choice should ideally be quick, readily available, and sensitive to detect hemorrhage in the subarachnoid space.

## Why the Correct Answer is Right
The best investigation of choice for diagnosing a subarachnoid hemorrhage, especially in an acute setting, is **Computed Tomography (CT) of the head without contrast**. This is because CT scans are highly sensitive for detecting acute blood in the subarachnoid space, with a sensitivity of nearly 100% within the first 24-48 hours after hemorrhage. Blood appears hyperdense on CT scans, making acute hemorrhage easily identifiable. Additionally, CT scans are quick, widely available, and can be performed rapidly, which is crucial in an emergency setting.

## Why Each Wrong Option is Incorrect
- **Option A:** While **Magnetic Resonance Imaging (MRI)** can be useful in detecting SAH, especially in the subacute or chronic phase when CT findings may be less apparent, it is not the first-line investigation due to its longer scanning time, lesser availability, and issues with claustrophobia or monitoring unstable patients.
- **Option B:** **Lumbar puncture (LP)** can be used to diagnose SAH when CT is negative but clinical suspicion remains high. However, LP is invasive and carries a risk of precipitating brain herniation if there's a mass effect. It's also less specific and sensitive than CT for acute blood.
- **Option D:** **X-ray of the skull** is not useful in the acute diagnosis of SAH. While it might show signs suggestive of increased intracranial pressure or fractures, it cannot directly visualize the hemorrhage.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that if the CT head is negative but the clinical suspicion for SAH remains high, a lumbar puncture should be performed to look for blood or bilirubin in the cerebrospinal fluid. However, the initial go-to imaging should always be a **non-contrast CT head** due to its speed and sensitivity for acute blood.

## Correct Answer: C. CT head without contrast.