A healthy 69-year-old man with osteoarthritis presented with darkening of the skin

This patient had left knee pain. What is the diagnosis


A healthy 69-year-old man with osteoarthritis of the knees presented with a 2-week history of darkening of the skin on the left thigh. On examination, there was a reddish-brown, pruritic, nontender, macular, nonblanching discoloration around the medial aspect of his left knee, with a few superficial erosions. He had no fever, chills, or other constitutional symptoms. For several weeks before this event, the patient had applied a heating pad repeatedly to his left knee to relieve the discomfort from the osteoarthritis.

What is the diagnosis?

Acanthosis nigricans
Erythema ab igne
Mycosis fungoides
Livedo reticularis



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Sumaiya Malik says:

erythema ab igne.