What Could Be the Cause of This Rash?

Answer: Erythema ab igne

This patient was using a “field corn pad” to relieve her lower back pain. The heat from the pad produced the livedo or vascular pattern seen here. The injury is believed to be infrared injury to the reticular arteriole network in the skin. 

Differential diagnosis. Cutis marmorata has a reticular pattern but is present from birth. Erythema annulare centrifugum is typically scaly. Granuloma annulare usually has discrete annulare lesions and is not as dusky as seen here. Livedo reticularis is typically seen on the lower extremities so finding it confined just to the lower back would not be expected.

Bonus question: What is “field” corn and how does it differ from sweet corn?

Answer: Field corn is typically used to feed livestock. It is not nearly as sweet as sweet corn, so it does not taste very good right off the cob. Field corn can be made into corn meal or corn flour, and then used in foods that have corn as an ingredient such as tortilla chips.