This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cornea and sclera, Subject name: Ophthalmology
Safe size of corneal graft with less chances of failure is - 
Select and explain. 
A. 7.5 mm, B. 5.5 mm, C. 6.5 mm, D. 4.5 mm
A: 7.5 mm
Answer: A: 7.5 mm 
Explanation: With the help of a corneal trephine (7.5 mm to 8 mm in size)a paial thickness incision is made in the hostcornea . Then, anterior chamber isentered with the help of a razor blade knife andexcision is completed using corneo-scleral scissors. REF:Comprehensive ophthalmology,AK Khurana,4th edition,pg no.125.