**Core Concept**
The power of an intraocular lens (IOL) is calculated to achieve optimal vision post-cataract surgery. This involves considering the patient's refractive error, axial length of the eye, and other factors. The calculation of IOL power is a critical step in ensuring the best possible visual outcome.

**Why the Correct Answer is Right**
The power of an IOL is typically calculated using the following formula: IOL power (D) = (Refraction (D) + (0.9 x Axial Length (mm))) / (Length of the IOL in the eye). The correct calculation involves using the patient's refractive error, axial length of the eye, and the length of the IOL in the eye. The formula is a modification of the SRK (Sanders, Retzlaff, and Kraff) formula, which takes into account the axial length of the eye.

**Why Each Wrong Option is Incorrect**
**Option A:** **Ultrasound** is actually required to measure the axial length of the eye, which is a crucial parameter in calculating the power of the IOL.
**Option B:** **Pachymetry** is a technique used to measure the thickness of the cornea, which can affect the calculation of IOL power, particularly in eyes with significant corneal astigmatism.
**Option C:** **Ophthalmometer** is an instrument used to measure the curvature of the cornea and the length of the eye, both of which are necessary for calculating IOL power.

**Clinical Pearl / High-Yield Fact**
A common pitfall in IOL power calculation is neglecting to account for significant corneal astigmatism, which can lead to suboptimal visual outcomes. It's essential to consider the patient's full refractive error, including astigmatism, when calculating IOL power.

**Correct Answer: A. Ultrasound**