gout

Your flatmate has dermatographism, which is a skin response to physical pressure. The rash that appears is similar to the rash that occurs when you come into contact with nettles. Nettle rash is caused by a chemical contained in the plant. Dermatographism is caused by the pressure of the chopstick on the skin. About 2-5 per cent of the population has dermatographism. It is a form of urticaria, which is a family of conditions that cause skin rashes. People who have urticaria tend to develop raised, itchy red rashes in response to a variety of stimuli. Sometimes the stimulus is chemical, as with nettle rash. But some people will respond in the same way to simple physical pressure on the skin. The redness is caused by the release of the chemical histamine.