Diagnosis
Stagnant water in the evaporator drain pan is promoting bacterial growth. This pan is located under the refrigerator and is not self-cleaning.
How to Fix It
Remove and clean the pan. Unplug the refrigerator. Pull it forward. Remove the toe grille and the front access panel. Slide out the black plastic drain pan. Wash it with warm, soapy water and a 10% bleach solution. Dry thoroughly before reinstalling.
If That Didn't Work
The smell may originate from the drain tube itself. Pour 1/4 cup of a baking soda and warm water mixture down the drain tube (located inside the refrigerator compartment, at the back wall near the bottom).
Heads Up
Wear gloves. The pan water can be contaminated. Reinstall the pan perfectly level. An angled pan will not catch all condensate, leading to leaks under the unit.