Answer: C - Atypical teratoid/rhabdoid tumor, WHO Grade 4

Atypical teratoid/rhabdoid tumor (AT/RT) is an aggressive primary CNS malignancy. Similar to  to renal and extrarenal rhabdoid tumors, these are tumors of the very young, with most cases being diagnosed within the first 3 years of life. 

Nearly all AT/RTs are intracranial, occurring in the supratentorial and posterior fossa compartments in approximately equal frequencies. Within these regions, they are most commonly found intracerebrally and cerebellum/cerebellopontine angle, respectively.  Rarely, they can appear to arise from the ventricular system, as in this case. 

While AT/RTs are associated with a classic rhabdoid morphology, these are generally undifferentiated (high-grade) tumors that can simulate several cell lineages both histologically and immunohistochemically. This can include variable amounts of epithelial (EMA, CK), glial (GFAP/S100), primitive neuronal (synaptophysin), and mesenchymal (SMA/desmin) areas. Although such features are highly suggestive of AT/RT,  definitive diagnosis requires loss of INI-1 nuclear expression (see image) or biallelic inactivation of the corresponding SMARCB1 gene. Rare cases can be caused by SMARCA4 inactivation, corresponding to BRG1 protein loss.