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
Shoening of 2 mm of axial length of eye ball causes ? 
Select and explain. 
A. 3D myopia, B. 6D myopia, C. 3D hypermetropia, D. 6D hypermetropia
D: 6D hypermetropia
Answer: D: 6D hypermetropia 
Explanation: Ans. is. d., 6D hypermetropia Change in axial length can cause refractive error. It is the commonest form of ametropia (both myopia and hypermetropia). In hypermetropia, there is an axial shoening of eyeball. So, image is formed behind the retina. In myopia, there is an axial lengthening of eyeball. So, image is formed in front of the retina. 1 mm change in axial length leads to ametropia of 3D. For example 1 mm shoening in axial length causes hypermetropia of 3D.