**Core Concept**
The Glasgow Coma Scale (GCS) is a neurological scale used to assess the level of consciousness in a person following a head injury. It evaluates the patient's eye opening, verbal response, and motor response to provide a score that correlates with the severity of brain injury.

**Why the Correct Answer is Right**
The GCS is widely recognized as the best prognostic factor for head injury because it provides a standardized and objective assessment of the patient's neurological status. The scale is divided into three components: eye opening (score 1-4), verbal response (score 1-5), and motor response (score 1-6). The total score ranges from 3 to 15, with higher scores indicating better neurological function. A score of 8 or less is generally associated with a poor prognosis, while a score of 9-12 is considered moderate, and a score of 13-15 is associated with a good outcome. The GCS is a reliable predictor of patient outcome and is used by clinicians to guide treatment decisions and predict the likelihood of recovery.

**Why Each Wrong Option is Incorrect**
**Option A:** Age of the patient is not a reliable prognostic factor for head injury, as outcomes can vary significantly among patients of the same age. While age can influence the severity of injury, it does not correlate with outcome as accurately as the GCS.
**Option C:** The mode of injury is an important factor in determining the severity of head injury, but it is not a reliable prognostic factor. For example, a patient who suffers a severe head injury from a low-impact fall may have a worse outcome than a patient who suffers a less severe injury from a high-impact collision.
**Option D:** The presence of facial trauma can be associated with head injury, but it is not a reliable prognostic factor. Facial trauma can occur in isolation from head injury, and the presence of facial trauma does not necessarily correlate with the severity of brain injury.

**Clinical Pearl / High-Yield Fact**
The GCS is a critical component of the initial assessment of patients with head injury, and its score should be documented at regular intervals to monitor changes in neurological status. A decrease in GCS score over time is a sign of deteriorating neurological function and may indicate the need for urgent intervention.

**✓ Correct Answer: B. Glasgow coma scale**