Mottled Darkening of the Skin on the Left Thigh

A healthy 69-year-old man with osteoarthritis of the knees presented with a 2-week history of mottled darkening of the skin on the left thigh.

On examination, there was a reticular, 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.

What is the most likely diagnosis?