**Core Concept**
Infantile hydrocephalus is often caused by an obstruction in the cerebrospinal fluid (CSF) pathway, leading to an accumulation of CSF in the brain and subsequent increase in intracranial pressure. This obstruction can be due to various factors, including congenital anomalies, infections, or tumors.

**Why the Correct Answer is Right**
The most common cause of hydrocephalus in infants is a congenital aqueductal stenosis, also known as the Dandy-Walker malformation. This condition is characterized by a narrowing or blockage of the aqueduct of Sylvius, which is a narrow channel that connects the third and fourth ventricles in the brain. As a result, CSF cannot flow through the aqueduct, leading to an accumulation of CSF in the lateral ventricles and subsequent hydrocephalus.

**Why Each Wrong Option is Incorrect**
**Option A:** Arachnoid cysts are collections of fluid between the arachnoid mater and the dura mater, but they are not typically associated with hydrocephalus in infants.
**Option B:** Meningitis can cause hydrocephalus, but it is not the most common cause in infants.
**Option C:** Tumors can cause hydrocephalus, but they are relatively rare in infants.

**Clinical Pearl / High-Yield Fact**
The Dandy-Walker malformation is a classic cause of infantile hydrocephalus, and it is often associated with other congenital anomalies, such as microcephaly, developmental delay, and seizures.

**Correct Answer: C. Tumors can cause hydrocephalus, but they are relatively rare in infants.