Pityriasis rosea (say “pih-tih-RY-uh-sus ROH-zee-uh”) is a common skin problem that causes a rash. The rash often begins with a herald patch-a single, round-to-oval, pink patch that is scaly with a raised border. The size of the patch ranges from 2 cm (0.8 in.) to 10 cm (3.9 in.).