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
Which of the following instrument is not required to calculate the power of IOL? 
Select and explain. 
A. Amplitude scan, B. Keratometer, C. Pachymetry, D. Biometer
C: Pachymetry
Answer: C: Pachymetry 
Explanation: Amplitude scan gives Axial length.
Keratometer gives corneal curvature.
Biometer is used to calculate the power of IOL.
Axial length and corneal curvature are substituted in SRK formula for calculation of IOL.
Pachymetry is used to calculate thickness of Cornea which is not used in SRK formula for calculation of IOL.