**Core Concept**
Hydrocephalus is a condition characterized by an accumulation of cerebrospinal fluid (CSF) within the brain, leading to increased intracranial pressure and enlargement of the ventricles. In infants, hydrocephalus can be congenital or acquired, and the underlying cause often determines the site of obstruction.

**Why the Correct Answer is Right**
The most common cause of congenital hydrocephalus is a blockage at the level of the aqueduct of Sylvius, which is a narrow passage connecting the third and fourth ventricles. This obstruction prevents the normal flow of CSF from the third to the fourth ventricle, leading to a buildup of fluid in the lateral ventricles. The aqueduct of Sylvius is a critical site for CSF circulation, and its obstruction is a common cause of hydrocephalus in infants.

**Why Each Wrong Option is Incorrect**
**Option A:** Obstruction at the foramen of Monro is less common in congenital hydrocephalus, as it is more often associated with acquired causes such as tumors or infection.

**Option B:** Obstruction at the level of the fourth ventricle is rare in infants, as it is more often associated with conditions such as Dandy-Walker malformation.

**Option C:** Obstruction at the level of the third ventricle is not a typical cause of congenital hydrocephalus, as it is more often associated with acquired causes such as tumors or infection.

**Clinical Pearl / High-Yield Fact**
The aqueduct of Sylvius is a critical site for CSF circulation, and its obstruction is a common cause of hydrocephalus in infants. This is an important consideration for pediatric neurosurgeons and radiologists when diagnosing and managing congenital hydrocephalus.

**Correct Answer:** . Aqueduct of Sylvius