What causes a brown discoloration on my wife's lower legs?

Pigmentary changes: Difficult to say without seeing. Commonly brown discoloration can occur over the skin after inflammation has subsided and healing is occuring in areas of previous rashes. This phenomenon is known as post-inflammatory hyperpigmentation (pih). On lower legs this can occur after developing chronic stasis dermatitis. Pih usually improves with time.

Many possibilities: There are many things that may cause discoloration of the skin. Several types of inflammatory conditions of the skin, poor venous or arterial circulation, allergic reactions, bacterial, viral or fungal infections, insect bites, friction, and cancer can all cause discoloration of the skin. Have it evaluated by a professional and get the appropriate treatment.