The third ventricle and both lateral ventricles are enlarged in a CT scan of a newborn infant's head. The most likely site of blockage of CSF flow is the

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The cerebral aqueduct, also known as the aqueduct of Sylvius, connects the third ventricle to the fourth ventricle. If it is blocked, CSF cannot flow from the lateral and third ventricles to the fourth ventricle and then to the subarachnoid space, leading to enlargement of the lateral and third ventricles.

The Third Ventricle and Both Lateral Ventricles Are Enlarged in a CT