This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.N.S., Subject name: Medicine
[{'content': 'Sea saw nystagmus is seen in : \nSelect and explain. \nA. Anterior longitudinal fasiculus, B. Craniopharyngioma, C. Medullary lesion, D. G.B. Syndrome', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' Craniopharyngioma (Ref Parson, 18/e, p 336).In Parson it is given that sea-saw nystagmus occurs due to para chiasmatic masses and the only para chiasmatic mass in the option is craniopharyngioma.", 'role': 'assistant'}]