MRI of a 6-year-old boy with progressive morning headaches shows marked dilatation of both lateral ventricles and the third ventricle, with a normal-sized fourth ventricle. The most likely site of obstruction is:

• A Foramen of Monro
• B Arachnoid villi
• C Foramina of Luschka and Magendie
• D Aqueduct of Sylvius ✓

Explanation

Obstruction proximal to the fourth ventricle produces noncommunicating hydrocephalus limited to the chambers above the block. Aqueductal stenosis dilates the lateral and third ventricles while sparing the fourth. A foramen of Monro lesion would dilate only the ipsilateral lateral ventricle, and obstruction at the fourth ventricle outflow would enlarge the entire ventricular system including the fourth ventricle. Impaired arachnoid villi absorption gives communicating hydrocephalus with uniform enlargement.

Reference: Ganong's Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.