## Core Concept
Non-communicating hydrocephalus, also known as obstructive hydrocephalus, occurs when there is a blockage in the cerebrospinal fluid (CSF) pathway within the ventricular system or at the level of the fourth ventricle's outlets. This blockage prevents CSF from flowing freely, leading to increased intracranial pressure and ventricular dilation. In children, the causes of non-communicating hydrocephalus can be diverse, including congenital anomalies, tumors, and acquired conditions.

## Why the Correct Answer is Right
The correct answer, **Congenital aqueductal stenosis**, is a common cause of non-communicating hydrocephalus in children. This condition involves a narrowing or blockage of the cerebral aqueduct, which connects the third and fourth ventricles and is a critical pathway for CSF flow. Congenital aqueductal stenosis can occur due to genetic factors, intrauterine infections, or developmental anomalies. It often presents in infancy or early childhood with signs of increased intracranial pressure and ventricular enlargement.

## Why Each Wrong Option is Incorrect
- **Option A:** While tumors can cause obstructive hydrocephalus by compressing or invading the brain structures involved in CSF circulation, they are less common in children compared to congenital anomalies and are not the most frequent cause.
- **Option B:** Post-hemorrhagic hydrocephalus usually results from a communicating type of hydrocephalus due to the blockage of CSF absorption through the arachnoid villi by blood products.
- **Option C:** Although infections can lead to hydrocephalus, they more commonly cause communicating hydrocephalus through mechanisms like meningitis leading to arachnoiditis and impaired CSF absorption.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **congenital aqueductal stenosis** often presents with a classic triad of symptoms: macrocephaly (enlarged head), sunset eyes (downward gaze palsy), and a characteristic "setting sun" sign. Early recognition and intervention, such as CSF diversion procedures (e.g., ventriculoperitoneal shunt), are critical to prevent long-term neurological damage.

## Correct Answer: D. Congenital aqueductal stenosis