**Core Concept**
Hydrocephalus is a condition characterized by the accumulation of cerebrospinal fluid (CSF) within the brain, leading to increased intracranial pressure and subsequent enlargement of the head.

**Why the Correct Answer is Right**
Hydrocephalus occurs when there is an imbalance between the production and absorption of cerebrospinal fluid, resulting in its accumulation within the brain's ventricles. This can be caused by a variety of factors, including congenital malformations, infections, or tumors. The increased CSF pressure and volume lead to the expansion of the cranial vault, causing the head to enlarge. In infants and children, this is often the most visible manifestation of hydrocephalus.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteogenesis imperfecta is a genetic disorder characterized by brittle bones, often resulting in fractures and bowing of the long bones. While it can cause skeletal abnormalities, it is not typically associated with macrocephaly.

**Option B:** Mucopolysaccharidosis is a group of genetic disorders caused by the deficiency of enzymes involved in the degradation of mucopolysaccharides. These disorders can lead to a range of systemic symptoms, including skeletal abnormalities and organomegaly, but are not typically characterized by macrocephaly.

**Option D:** Cerebral gigantism, also known as Sotos syndrome, is a rare genetic disorder characterized by excessive growth during childhood, leading to macrocephaly and other skeletal abnormalities. However, it is a distinct clinical entity from hydrocephalus and is not typically associated with the same underlying pathophysiology.

**Clinical Pearl / High-Yield Fact**
In infants and children, the presence of a large head should prompt a thorough evaluation for hydrocephalus, including imaging studies such as ultrasound or CT scans. Early detection and treatment of hydrocephalus can significantly improve outcomes and prevent long-term sequelae.

**✓ Correct Answer: C. Hydrocephalus**