###Rationale: Ans. is 'a' i.e., Nystagmus Illuminated frenzel galsses (+20 lenses) are useful for abolishing fixation and thus revealing peripheral vestibular nystagmus.

###Answer: OPTION A IS CORRECT.
###Question: Illuminated frenzel glasses are used in detecting?
###Options:
A. Nystagmus
B. Heterophoria
C. Esotropia
D. Astigmatism
