What instrument is used to examine the interior portion of the eye?

The instrument used to examine the interior portion of the eye is the ophthalmoscope. This tool allows healthcare providers to inspect the retina and other internal eye structures by shining a light through the pupil. Through the combination of lenses and light, an ophthalmoscope enables a clear view of the optic nerve head, blood vessels, and the macula, which are crucial in diagnosing various ocular conditions such as diabetic retinopathy, glaucoma, and macular degeneration.

In contrast, a stethoscope is primarily used for listening to heart and lung sounds, and a tonometer is specifically designed to measure intraocular pressure for glaucoma screening. A retinoscope, while used for refracting errors in vision and examining the back of the eye, does not provide as comprehensive a view of the interior structures as an ophthalmoscope does. Thus, the ophthalmoscope stands out as the correct choice for examining the interior portion of the eye.