What instrument is specifically used to examine the ciliary body, the periphery of the retina, and the angle structure?

The instrument specifically designed for the examination of the ciliary body, the periphery of the retina, and the angle structure is a goniolens. This specialized lens enables a detailed view of the anterior chamber angles, allowing the clinician to assess for conditions such as glaucoma, as well as to evaluate the health and structure of the ciliary body and peripheral retina. The goniolens works by providing a wider field of view and creating an appropriate angle to visualize these critical areas that are usually difficult to examine directly.

Other instruments listed serve different functions: the Maddox rod is primarily used for assessing ocular alignment and detecting heterophoria; the keratometer measures the curvature of the cornea, which is essential for determining the fit of contact lenses but unrelated to the structures mentioned; and the pachymeter is used to measure corneal thickness, which is relevant to conditions like glaucoma but does not focus on the ciliary body or peripheral retina. The specificity of the goniolens for examining the anterior segment structures highlights its importance in ophthalmic evaluations, making it the correct choice.