<prompt> Down beat nystagmus could be due to-a Cerebellar lesionb Arnold - Chiari malformationc Optic neuritisd Pontine lesionacaabbc  <response>: Upbeat nystagmus lesions of central tegmentum of brain stein.Downbeat nystagmus Cerebellar lesions and Arnold chiari syndrome. It is due to posterior fossa disease and compression at foramen magnum level. 