**Core Concept**
In the context of an unconscious patient with diabetes mellitus and a history of atrial fibrillation on warfarin, the primary concern is to rule out bleeding as a cause of altered mental status. Warfarin, a vitamin K antagonist, increases the risk of bleeding due to its anticoagulant effects.

**Why the Correct Answer is Right**
Given the patient's history of diabetes mellitus and warfarin use, the first step in the workup would be to investigate for bleeding as a cause of altered mental status. A non-contrast head CT would be the preferred imaging modality to quickly assess for intracranial hemorrhage (ICH) without the risk of contrast-induced nephropathy, which is a concern in patients with renal impairment, often seen in diabetic patients. Furthermore, a non-contrast head CT is more sensitive and specific for detecting acute hemorrhage compared to other imaging modalities.

**Why Each Wrong Option is Incorrect**
* **Option A:** Lumbar puncture (LP) would not be the first-line investigation in this patient due to the risk of precipitating or exacerbating a potential subarachnoid hemorrhage, which could lead to a fatal outcome.
* **Option B:** MRI might be useful in detecting other types of intracranial pathology, such as ischemic strokes or tumors, but it would not be the initial investigation of choice in a patient with a high suspicion of bleeding.
* **Option D:** CT angiography might be useful in detecting vascular abnormalities or aneurysms, but it would not be the initial investigation of choice in a patient with a high suspicion of bleeding.

**Clinical Pearl / High-Yield Fact**
In a patient with altered mental status and a history of anticoagulation, a non-contrast head CT is the preferred imaging modality for quickly assessing for intracranial hemorrhage.

**Correct Answer: A. Lumbar puncture would not be done in this patient for workup.**