Derm Dx: A necrotic mass on the back

A 57-year-old patient presented with a large, oblong necrotic mass on the left posterior lateral region of his back.

The lesion was 35 cm x 16 cm, tender, but firm with an erythematous border. His last visit was six years prior, during which an inflamed cyst on the mid portion of his back was treated with an intralesional injection of ...