**Core Concept:**
Headache is a common symptom in clinical practice, and it can be caused by various pathological processes within the brain or peripheral structures. Differentiating between primary headache disorders (e.g., migraine, tension headache) and secondary headache disorders (e.g., sinusitis, subarachnoid haemorrhage) is crucial for appropriate diagnosis and management.

**Why the Correct Answer is Right:**
In this scenario, the patient presents with a severe headache, which is a concerning symptom. The correct answer, option D - "Computed Tomography (CT) scan", is chosen due to its ability to visualize the brain and its surrounding structures, enabling the detection of intracranial pathology. CT scan is more sensitive for detecting intracranial hemorrhage, infarction, or space-occupying lesions compared to the other options.

**Why Each Wrong Option is Incorrect:**
A. Magnetic Resonance Imaging (MRI): Although MRI is a valuable diagnostic tool, it is not the first-line investigation for a patient with a severe headache. CT scan is more suitable in this case due to its speed, accessibility, and higher sensitivity in detecting intracranial pathology.
B. Electrocardiogram (ECG): An ECG is essential for cardiac evaluation, but it is irrelevant in this scenario, as it does not address the neurological symptoms presented by the patient.
C. Blood tests: While laboratory tests are essential for evaluating systemic diseases or electrolyte imbalances, they are not specific for a severe headache and do not directly address the patient's neurological symptoms.

**Clinical Pearl:**
In clinical practice, it is crucial to consider the age, history, and presentation of symptoms when choosing the most appropriate diagnostic test. A severe headache in a patient of this age group should raise suspicion for intracranial pathology, and CT scan is the most suitable imaging modality in this context due to its higher sensitivity and immediate availability compared to MRI.

**Correct Answer:** Computed Tomography (CT) scan.