This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Optics and Errors of Refraction, Subject name: Ophthalmology
[{'content': 'Intrastromal corneal ring segments (intacs) are used in all except \nSelect and explain. \nA. Post LASIK keratectasia, B. Keratoconus, C. Low myopia, D. Irregular astigmatism', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Intacs are not used for irregular astigmatism. Intacs consists of 2 semicircular rings made of PMMA and are used to flatten the cornea. It does not disturb vision. Intacs require atleast 400 microns of corneal stromal thickness. They are used most impoantly for keratoconus. It is also used for low myopia -1 to -3 D. But it has no role in irregular astigmatism which requires contact lenses for correction.', 'role': 'assistant'}]