This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Lens, Subject name: Ophthalmology
Power of Intraocular lens is determined by? 
Select and explain. 
A. Keratometry, B. Retinoscopy, C. Ophthalmoscopy, D. Gonioscopy\/Biometry
A: Keratometry
Answer: A: Keratometry 
Explanation: The most common method of determining IOL power uses a regression formula called 'SRK (Sanders, Retzlaff and Kraff) formula'. The formula is P = A - 2.5L - 0.9K, where: P is the power of IOL,  A is a constant which is specific for each lens type.  L is the axial length of the eyeball in mm, which is determined by A-scan ultrasonography.  K is average corneal curvature, which is determined by keratometry.