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': 'Illuminated frenzel glasses are used in detecting? \nSelect and explain. \nA. Nystagmus, B. Heterophoria, C. Esotropia, D. Astigmatism', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Nystagmus Illuminated frenzel galsses (+20 lenses) are useful for abolishing fixation and thus revealing peripheral vestibular nystagmus.", 'role': 'assistant'}]