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
[{'content': 'Essential parameters for IOL power calculation are \nSelect and explain. \nA. Keratometry and axial length of the eyeball, B. Keratometry and corneal thickness, C. Corneal thickness and axial length of the eyeball, D. Corneal thickness and anterior chamber depth', 'role': 'user'}, {'content': "Answer: A \nExplanation: Calculation of IOL power (Biometry) : 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. The ultrasound machine equipped with A-scan and IOL power calculation software is called 'Biometer&;. Ref:- A K KHURANA; pg num:-196", 'role': 'assistant'}]