**Core Concept**
Hydrocephalus is a condition characterized by an accumulation of cerebrospinal fluid (CSF) within the brain, leading to increased intracranial pressure. In children, the first sign of hydrocephalus often involves changes in cranial morphology and neurological function.

**Why the Correct Answer is Right**
The first sign of hydrocephalus in children is often a visible change in the shape of the head, particularly a rapid increase in head circumference. This is due to the expanding intracranial pressure forcing the cranial bones to separate, resulting in a soft spot or bulge at the anterior fontanelle, also known as the "soft spot" on the baby's head. As the condition progresses, the increased intracranial pressure can lead to damage to the brain tissue, causing a range of neurological symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Maculopapular rash - This is not a typical sign of hydrocephalus in children.
**Option B:** Diarrhea - This is not related to hydrocephalus.
**Option C:** Seizure - While seizures can be a symptom of hydrocephalus, they are not the first sign in children.

**Clinical Pearl / High-Yield Fact**
In infants and young children, a rapid increase in head circumference or a visible soft spot on the anterior fontanelle can be a crucial indicator of hydrocephalus, emphasizing the importance of regular head circumference measurements during pediatric check-ups.

**Correct Answer: D. Visible change in head shape.**