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
Down beat nystagmus could be due to-a) Cerebellar lesionb) Arnold - Chiari malformationc) Optic neuritisd) Pontine lesion 
Select and explain. 
A. ac, B. a, C. ab, D. bc
C: ab
Answer: C: ab 
Explanation: 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.