**Core Concept**
Increased intracranial pressure (ICP) following head injury can manifest through various clinical signs and symptoms, primarily due to the brain's compensatory mechanisms and the displacement of brain structures. The earliest manifestation of elevated ICP is often related to the brain's response to increased pressure, which can lead to impaired brain function.

**Why the Correct Answer is Right**
Altered mental status, including confusion, disorientation, and decreased level of consciousness, is the earliest manifestation of increased ICP following head injury. This is because elevated ICP can compress the brainstem and disrupt the normal functioning of the cerebral cortex. The brain's compensatory mechanisms, such as vasodilation and increased cerebrospinal fluid (CSF) production, attempt to alleviate the pressure, but these efforts are not always sufficient to prevent the development of altered mental status. The compression of the brainstem can also affect the pupillary light reflex, but this is typically a later manifestation of increased ICP.

**Why Each Wrong Option is Incorrect**
**Option A:** Ipsilateral pupillary dilatation is not typically an early manifestation of increased ICP following head injury. Ipsilateral pupillary dilatation may occur in cases of third nerve palsy or in the presence of a large ipsilateral mass lesion, but it is not a direct consequence of increased ICP.

**Option B:** Contralateral pupillary dilation is not a characteristic sign of increased ICP following head injury. While pupillary dilation may occur in the presence of increased ICP, it is typically ipsilateral to the mass lesion or third nerve palsy.

**Option D:** Hemiparesis, or weakness on one side of the body, is not an early manifestation of increased ICP following head injury. Hemiparesis may occur in cases of focal brain injury, such as a stroke or intracerebral hemorrhage, but it is not a direct consequence of increased ICP.

**Clinical Pearl / High-Yield Fact**
Remember that the earliest manifestation of increased ICP following head injury is often subtle and may be overlooked. Altered mental status, including confusion and decreased level of consciousness, should be considered a red flag for increased ICP, and prompt evaluation and management are essential to prevent further brain injury.

✓ Correct Answer: C. Altered mental status