**Core Concept**
Hydrocephalus is a medical condition characterized by an accumulation of cerebrospinal fluid (CSF) within the brain, leading to increased intracranial pressure and potential damage to brain tissue. This condition can result from various causes, including congenital defects, infection, or trauma.

**Why the Correct Answer is Right**
The first sign of hydrocephalus in children is often an increase in head circumference, which can be detected during routine check-ups. This is because the growing child's skull is able to accommodate the expanding CSF-filled ventricles, allowing the head to increase in size. As the child grows, the increased head circumference can be a subtle yet crucial indicator of hydrocephalus. This is often the first sign, as it can occur before more severe symptoms such as seizures, vomiting, or lethargy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too nonspecific and can be a symptom of many other conditions.
**Option B:** This option is incorrect because it is a more advanced symptom of hydrocephalus, often occurring after the initial increase in head circumference.

**Clinical Pearl / High-Yield Fact**
It is essential for pediatricians to monitor head circumference in infants and young children, as early detection of hydrocephalus can significantly improve outcomes.

**Correct Answer: C. Increase in head circumference.**