Psoriasis is a common skin condition that causes skin cells to grow too quickly and build up, leaving thick, red, silvery, or scaly patches (plaques) on the surface of the skin. They stimulate skin cells to grow too quickly, and this causes the thick, scaly build-up seen in psoriasis plaques. Psoriasis (pronounced: suh-RYE-uh-sus) is a disease that causes skin cells to build up on the surface of the skin. The dead skin and white blood cells can’t be shed quickly enough. They build up on the surface of the skin as thick red patches.