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
Illuminated frenzel glasses are used in detecting? 
Select and explain. 
A. Nystagmus, B. Heterophoria, C. Esotropia, D. Astigmatism
A: Nystagmus
Answer: A: Nystagmus 
Explanation: Ans. is 'a' i.e., Nystagmus Illuminated frenzel galsses (+20 lenses) are useful for abolishing fixation and thus revealing peripheral vestibular nystagmus.