**Core Concept**
The underlying principle in this question is the assessment and management of patients with head injuries, which involves a comprehensive evaluation to identify potential complications such as intracranial hemorrhage, elevated intracranial pressure, or cerebral edema. The investigation of choice should provide rapid and accurate information to guide clinical decision-making.

**Why the Correct Answer is Right**
The investigation of choice for a patient with a head injury is a **non-contrast computed tomography (CT) scan**. This imaging modality is preferred because it is rapid, widely available, and sensitive for detecting acute hemorrhage, fractures, and other abnormalities that may require immediate surgical intervention. The non-contrast CT scan can be performed quickly, often within 10-15 minutes, which is critical in the acute setting where timely diagnosis and treatment can significantly impact patient outcomes.

**Why Each Wrong Option is Incorrect**
* **Option A:** Skull X-ray is outdated and not sensitive for detecting acute hemorrhage or other intracranial abnormalities.
* **Option B:** Magnetic Resonance Imaging (MRI) is more sensitive for detecting cerebral edema and other subtle changes, but it is not the initial imaging modality of choice due to its longer scanning time and lower availability.
* **Option C:** Angiography may be useful in specific cases, such as suspected vascular injury, but it is not the primary investigation for head injuries.

**Clinical Pearl / High-Yield Fact**
In the acute setting, a non-contrast CT scan should be performed as quickly as possible to identify potential complications, and the patient should be monitored closely for signs of increased intracranial pressure, such as headache, vomiting, or altered mental status.

**Correct Answer:** A. Non-contrast computed tomography (CT) scan.