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
Placido disc is used for diagnosing which of the following condition? 
Select and explain. 
A. Uveitis, B. Keratoconus, C. Retinoblastoma, D. Retinal detachment
B: Keratoconus
Answer: B: Keratoconus 
Explanation: Placido disc examination is used to diagnose keratoconus. In a case of keratoconus, placido disc examination shows irregularities of the circles. Placido's keratoscopic disc: It is a disc painted with alternating black and white circles. It may be used to assess the smoothness and curvature of corneal surface. Normally, on looking through the hole in the center of disc a uniform sharp image of the circle is seen on the cornea. Distoion of the circles occur when irregularities are present on the corneal surface. Ref: Comprehensive Ophthalmology By AK Khurana, 4th edn, page 119