The term 'cause of an itch' can be answered by two four-letter crossword solutions: FLEA and BITE. Fleas are small, wingless insects that feed on the blood of mammals and birds. They can cause itching due to their saliva, which contains an anticoagulant that can cause an allergic reaction in some people. Bites from other insects, such as mosquitoes, can also cause itching due to the body's reaction to the saliva of the insect.