**Core Concept:**
The question is asking about the appropriate investigation for a patient presenting with severe headache. A severe headache can be due to various causes, including intracranial haemorrhage, cerebral infarction, meningitis, subarachnoid haemorrhage, and migraine. The goal is to identify the underlying cause and initiate appropriate management.

**Why the Correct Answer is Right:**
The correct answer is **Option D** - **CT scan of the brain**. Computed tomography (CT) scan is a rapid and non-invasive imaging technique that can detect acute intracranial lesions, including haemorrhages, infarcts, and meningitis. It provides high-resolution images of the brain and skull, allowing for accurate assessment of the structures and potential abnormalities. In contrast to MRI, CT scans are quicker, more readily available, and better suited for patients with claustrophobia or pacemakers.

**Why Each Wrong Option is Incorrect:**

**Option A** - **MRI**: MRI (Magnetic Resonance Imaging) is also a valuable imaging modality, but it may not be suitable in some patient situations, such as claustrophobia or the presence of certain implants or medical devices.

**Option B**: **CTA (Computed Tomography Angiography)** and **MRA (Magnetic Resonance Angiography)** are specialized CT and MRI techniques, respectively, used for evaluating blood vessels. In this case, the patient's severe headache suggests an intracranial pathology rather than a vascular disorder, making these options less appropriate.

**Option C**: **Lumbar puncture (Spinal Tap)** is an invasive procedure primarily used for diagnosing meningitis and other conditions involving the cerebrospinal fluid (CSF). It is less relevant in this case, as there is no mention of meningitis or any other CSF-related condition.

**Core Concept:**
The choice of imaging modality depends on the patient's clinical presentation, medical history, and the specific suspected diagnosis. In this scenario, a non-invasive and readily available imaging modality like CT scan is more appropriate to rapidly detect intracranial pathology.

**Why the Correct Answer is Right:**

**Option D** - **CT scan of the brain** is a reliable and widely available diagnostic tool that can promptly identify a variety of intracranial lesions, including haemorrhages, infarcts, and space-occupying lesions. It is essential to rule out these conditions in a patient with severe headache, especially when other options like MRI or lumbar puncture are less pertinent due to the absence of meningitis or CSF-related concerns.

**Why Each Wrong Option is Incorrect:**

**Option A** - **MRI**: Although MRI is a valuable imaging modality for neuro-imaging, its use is not justified in this context since the patient's chief complaint is severe headache, not vascular or CSF-related issues.

**Option B**: **CTA (Computed Tomography Angiography)** and **MRA (Magnetic Resonance Angiography)** are radiological techniques used for evaluating blood vessels. In this case, the patient's presentation is primarily concerning