## **Core Concept**
The question describes a condition in a 5-day-old infant male with an abnormally large head due to enlarged lateral and third ventricles but a normal-size fourth ventricle. This presentation suggests a form of hydrocephalus caused by an obstruction in the cerebrospinal fluid (CSF) pathway. The cerebral aqueduct (of Sylvius) connects the third and fourth ventricles, and its stenosis would lead to this specific pattern of ventricular enlargement.

## **Why the Correct Answer is Right**
The characteristic symptoms and imaging findings point towards hydrocephalus due to aqueductal stenosis. This condition leads to an accumulation of CSF proximal to the obstruction, causing enlargement of the lateral and third ventricles. The fourth ventricle and the subarachnoid space distal to the obstruction remain unaffected, explaining the normal size of the fourth ventricle. This specific pattern of ventricular enlargement is a hallmark of obstructive (or non-communicating) hydrocephalus due to aqueductal stenosis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not specifically relate to the described pathophysiology of hydrocephalus due to aqueductal stenosis.
- **Option B:** This option is incorrect as it does not accurately represent the clinical condition described.
- **Option C:** This option might seem plausible but does not directly correlate with the specific findings of enlarged lateral and third ventricles with a normal fourth ventricle due to aqueductal stenosis.
- **Option D:** This option is incorrect because it does not match the clinical presentation and the suspected diagnosis of hydrocephalus caused by stenosis of the cerebral aqueduct.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that hydrocephalus due to aqueductal stenosis often presents with signs of increased intracranial pressure and macrocephaly in infants, such as a bulging fontanelle, separated cranial sutures, and an increased head circumference. Early recognition and intervention, such as CSF diversion procedures (e.g., ventriculoperitoneal shunt), are crucial to prevent long-term neurological damage.

## **Correct Answer:** . Hydrocephalus.