Interesting article from NEJM. It is a common MRCP question for Part 1 & 2a. Do not forget its relationship with hypothyroidism.
This patient has L knee pain.
This reticular, reddish-brown, pruritic, nontender, macular, nonblanching discoloration around the medial aspect of the left knee, with a few superficial erosions, is most consistent with erythema ab igne. This patient had repeatedly applied a heating pad to his left knee in the preceding weeks to relieve discomfort from osteoarthritis.
Read More: New Engl J Med 356;9:e8