**Core Concept**
Increased intracranial pressure (ICP) following head injury can manifest in various ways, but the earliest and most common sign is often related to the brain's compensatory mechanisms. As ICP rises, it can lead to compression of brain structures, disrupting normal brain function.

**Why the Correct Answer is Right**
Altered mental status is the earliest manifestation of increased intracranial pressure following head injury because it reflects the brain's initial attempt to compensate for the increased pressure. As ICP rises, the brain's cerebral perfusion pressure is maintained by autoregulation, which involves vasodilation of cerebral vessels to increase blood flow. However, when this compensation is overwhelmed, the brain's function begins to deteriorate, leading to altered mental status. This can manifest as confusion, disorientation, or decreased level of consciousness.

**Why Each Wrong Option is Incorrect**
**Option A:** Ipsilateral pupillary dilatation occurs with third nerve compression, which is a more advanced sign of increased ICP, often associated with herniation. This is not the earliest manifestation.
**Option B:** Contralateral pupillary dilatation is not a typical sign of increased ICP. Pupillary changes are more commonly associated with third nerve compression or other specific conditions.
**Option D:** Hemiparesis is a sign of focal brain injury or stroke, not necessarily the earliest manifestation of increased ICP. It may occur later in the course of the injury.

**Clinical Pearl / High-Yield Fact**
In the context of increased ICP, it's essential to remember that the brain's compensatory mechanisms can mask the true extent of the injury. Altered mental status is often the first sign of impending brain herniation, highlighting the need for prompt evaluation and intervention.

**✓ Correct Answer: C. Altered mental status**