Which component of an eye test measures the pressure inside the eye?

Tonometry is the correct answer because it specifically measures the intraocular pressure (IOP) within the eye. This measurement is essential in diagnosing and monitoring conditions such as glaucoma, where elevated pressure can lead to optic nerve damage and vision loss. The tonometer works by gently contacting the eye and measuring the resistance of the cornea when a certain force is applied, allowing for the calculation of the pressure within the eye.

Refraction, on the other hand, is the process used to determine the appropriate lens prescription by measuring how light is focused in the eye. Keratometry focuses on measuring the curvature of the cornea, which is important for fitting contact lenses and understanding corneal health, but does not assess internal pressure. Visual acuity measures the clarity or sharpness of vision, typically through the use of an eye chart, and is not concerned with pressure measurements.