This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Ophthalmology
[{'content': 'Down beat nystagmus could be due to-a) Cerebellar lesionb) Arnold - Chiari malformationc) Optic neuritisd) Pontine lesion \nSelect and explain. \nA. ac, B. a, C. ab, D. bc', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Upbeat nystagmus → lesions of central tegmentum of brain stein.\nDownbeat nystagmus → Cerebellar lesions and Arnold chiari syndrome. It is due to posterior fossa disease and compression at foramen magnum level.', 'role': 'assistant'}]