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: Anatomy
Lamina cribrosa is a modification of : 
Select and explain. 
A. Sclera, B. Choroid, C. Optic nerve sheath, D. Retina
A: Sclera
Answer: A: Sclera 
Explanation: The sclera (skleros=hard) is opaque and forms the posterior five-sixths of the eyeball. It is composed of dense fibrous tissue which is firm and maintains the shape of the eyeball. It is thickest behind, near the entrance of the optic nerve, and thinnest about 6 mm behind the sclerocorneal junction where the recti muscles are inserted. However, it is weakest at the entrance of the optic nerve. Here the sclera shows numerous perforations for passage of fibres of the optic nerve. Because of its sieve-like appearance this region is called the lamina cribrosa (crib=sieve).