What is the narrowest part of the ventricular system?

• A. Cerebral aqueduct (Of Sylvius)
• B. Foramen of Monro
• C. Fourth ventricle
• D. Third ventricle

Answer: (A)

The flow of cerebrospinal fluid through the ventricular system passes from the lateral ventricles into the third ventricle via the foramen of Monro, then continues through the narrow canal that connects the third and fourth ventricles—the cerebral aqueduct (aqueduct of Sylvius)—and onward to the fourth ventricle before reaching the subarachnoid space. This aqueduct is the tightest bottleneck in the pathway, so it is the narrowest part of the ventricular system. Because of its small diameter, even a slight blockage here can obstruct CSF flow and lead to hydrocephalus.

The foramen of Monro is indeed small, but it is a single opening linking the lateral ventricles to the third ventricle, not the narrowest segment of the CSF pathway. The third ventricle is a slit-like cavity, not a slender canal, and the fourth ventricle, though contoured, is larger in cross-section than the aqueduct.