The diagnosis: 4^th ventricular mass lesion; consistent with medulloblastoma with moderate supratentorial hydrocephalic changes.  

Differential diagnoses:

• ependymoma: usually in older children, arises from the floor of the 4^th ventricle, more heterogenous with hemorrhage and calcifications. Extension through the lateral foramina of Luschka "plastic tumor".  
• cerebellar juvenile pilocytic astrocytoma (JPA): lateral cerebellar cystic mass with enhancing mural nodule.
• choroid plexus papilloma: usually in the 3^rd ventricle in children and 4^th ventricle in adults. The tumor shows intense and homogeneous enhancement.
• exophytic dorsal pontine glioma: the origin from the dorsal pons with pontine enlargement and filling the 4th ventricle.  
• Atypical teratoid/rhabdoid tumor: in young children with almost typical imaging.