Final answer:
The sign indicative of increasing intracranial pressure following a head injury is dilated pupils unresponsive to light, which suggests serious brain dysfunction and requires immediate medical attention.
Step-by-step explanation:
In assessing a patient with a head injury, certain signs can indicate an increase in intracranial pressure. The appropriate response from the options provided is dilated pupils unresponsive to light. This finding is a classic clinical sign of rising pressure within the skull, potentially due to brain swelling, hematoma, or other causes related to traumatic injury. The dilation of pupils that do not respond to light indicates a compression of the third cranial nerve or more generalized brain stem dysfunction, which can be a life-threatening condition requiring immediate medical attention. Conversely, slurred speech could be related to a variety of neurological impairments, including increased intracranial pressure, but does not specifically indicate it. Decerebrate posturing refers to an abnormal body posture that can accompany severe brain damage, typically involving the brain stem, but also doesn't singularly diagnose increased intracranial pressure. Lastly, flexion of the extremities is more commonly seen in decorticate posturing and although serious, is not a definitive sign of increasing intracranial pressure.