What is the most likely diagnosis for an 80-year-old female with a gray ring on the edge of both irises, without visual changes or pain?

The presence of a gray ring on the edge of both irises in an elderly patient is most commonly indicative of arcus senilis. This condition is characterized by the deposition of lipid material in the corneal stroma, which manifests as a gray or white arc at the periphery of the cornea. While this condition is frequently seen in older adults and is often normal with aging, it can also be associated with hyperlipidemia.

The absence of visual changes and pain further supports the diagnosis of arcus senilis, as this condition typically does not interfere with vision or cause discomfort.

In contrast, pinguecula, while it can cause a yellowish-white growth on the conjunctiva, does not typically present as a gray ring on the iris and is usually more localized to the conjunctiva than the corneal periphery. Peripheral cataracts primarily lead to visual changes and would not typically be described as a gray ring without accompanying symptoms. Macular degeneration is characterized by visual loss and does not present with a gray ring on the iris, focusing instead on central vision impairment.

Thus, the described symptoms align most closely with arcus senilis, making it the most likely diagnosis in this case.