Medulloblastoma

A 7-year-old boy presents with a 3-week history of morning headache, projectile vomiting, and progressive gait ataxia. On examination, he has papilledema, truncal ataxia with titubation, dysmetria, and a positive Romberg sign. MRI brain with contrast demonstrates a well-defined midline mass marked **A** arising from the cerebellar vermis and filling the fourth ventricle. The mass is hypointense on T1, hyperintense-to-isointense on T2, shows heterogeneous avid enhancement, and demonstrates restricted diffusion on DWI. There is marked dilatation of the lateral and third ventricles with transependymal edema. Which of the following best describes the tumor marked **A** and its most characteristic feature?