Which instrument is primarily used to visualize the interior structures of the eye?

The instrument primarily used to visualize the interior structures of the eye is the ophthalmoscope. This device allows practitioners to examine the retina, optic disc, and other internal components of the eye. It works by shining a light into the eye and using a lens to magnify the view of the structures. This examination is crucial for identifying a variety of conditions, such as diabetic retinopathy, glaucoma, and macular degeneration.

The other choices serve different purposes: a sphygmomanometer measures blood pressure, a retinoscope is used to assess refractive errors by evaluating how light reflects off the retina, and a tonometer measures intraocular pressure to assess for conditions like glaucoma. Each of these instruments has its specific use in eye care, but only the ophthalmoscope is specifically designed for direct visualization of the interior eye structures.