Unilateral Skin Hardening...

An 11-year-old boy presented with progressive unilateral skin hardening on his right thigh and buttock for 7 years. Recently, the motion of his right limb joint was slightly limited. He was born to healthy parents and had no medical disease.
There was an ill-defined, stony-hard plaque involving his right thigh above the patella extending to his right buttocks w/ loss of skin elasticity. Affected skin had mild hypertrichosis & hyperpigmentation.