What are the key differences between hydrocephalus due to aqueductal stenosis and obstructive hydrocephalus due to a mass?

• A. Aqueductal stenosis leads to enlargement of lateral and third ventricles with a normal fourth ventricle; a posterior fossa mass can enlarge multiple ventricles and may alter fourth ventricle size
• B. Aqueductal stenosis causes only fourth ventricle enlargement; mass enlarges only lateral ventricles
• C. Both produce identical ventricular enlargement patterns
• D. Aqueductal stenosis reduces ventricles

Answer: (A)

The main idea is that where the CSF flow is blocked shapes which ventricles get bigger. With aqueductal stenosis, the blockage is between the third and fourth ventricles, so CSF backs up into the lateral and third ventricles. The fourth ventricle doesn’t accumulate excess CSF because the blockage is upstream of it, so it tends to stay normal in size. In contrast, a mass in the posterior fossa often obstructs CSF outflow at the outlets of the fourth ventricle or presses on nearby pathways, causing dilation of multiple ventricles, including the fourth.

So the pattern of which ventricles are enlarged helps distinguish the two: enlargement of the lateral and third ventricles with a normal fourth points to aqueductal stenosis, whereas dilation across all ventricles (often including the fourth) suggests obstruction related to a posterior fossa mass. The other descriptions don’t fit the typical CSF-flow dynamics: they either misstate which ventricles enlarge or claim no change or opposite changes in size.