Obstruction to the flow of CSF at the aqueduct of Sylvius will most likely lead to enlargement of -

CSF is produced by choroid plexus in the lateral and III ventricles. Then it flows through the aqueduct of Sylvius into the 4th ventricle and from there into the subarachnoid space to be absorbed by the arachnoid Obviously, any obstruction in the aqueduct of Sylvius will lead to enlargement of the proximal ventricles (i.e. both lateral and IIIrd ventricles).