An infant with progressive head enlargement undergoes CT, which shows dilated lateral ventricles and a dilated third ventricle, but a normal-sized fourth ventricle. The site of obstruction is most likely the:

• A Arachnoid villi
• B Superior sagittal sinus
• C Foramen of Luschka
• D Aqueduct of Sylvius ✓

Explanation

Dilation of the lateral and third ventricles with a normal fourth ventricle localizes obstruction to the aqueduct of Sylvius, producing non-communicating hydrocephalus because CSF cannot exit the third ventricle. Obstruction at the arachnoid villi or superior sagittal sinus impairs absorption and dilates all four ventricles equally, giving communicating hydrocephalus. Obstruction at the foramina of Luschka would enlarge the fourth ventricle.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.