A 48-year-old woman from Delhi is admitted with acute onset vertigo, nausea, and vomiting following a sudden severe headache. Neurological examination reveals nystagmus, truncal ataxia, and inability to perform rapid alternating movements. CT brain shows acute hydrocephalus with compression of the fourth ventricle by a midline cerebellar mass. Which of the following structures is most directly responsible for the observed ataxia and loss of coordination?