Three of the pairs below name a disorder and the body part it attacks, and one does not. Choose the pair that breaks the classification.

A cataract clouds the lens of the eye, jaundice follows trouble in the liver, and arthritis inflames the joints, so each of those three names an ailment and the part it strikes. Enamel is no ailment at all — it is the hard white covering of a healthy tooth. A solver who narrows the shared rule to ailment and organ may be drawn to Arthritis : Joint, since a joint is not an organ, but a joint is still a body part and arthritis is still a disorder, so that pair keeps the rule.