A Kayser-Fleischer ring is a circular discoloration of the outer ring of the iris caused by an accumulation of copper. They are almost always a result of Wilson's disease.
Although easy to spot in a patient in the advanced stages of the disease, they are more difficult to spot in the early stages and often require careful examination. They are often discovered when looking for other conditions. Since the rings cause no secondary symptoms, such as vision loss, they are often easily overlooked during a routine examination. The general course of development is that they form a crescent in the upper half of the iris before spreading to the lower half.
A differential for Kayser-Fleischer ring starts with Wilson's, but if that can be ruled out, it is almost certainly due to either an obstruction of the bile duct or cirrhosis