Sudden swelling of a single finger can be caused by infection, allergy, trauma, tenosynovitis, arthritis and so on. 1. Infection: If a single finger is infected by bacteria or viruses, sudden swelling may occur, accompanied by pain and elevated skin temperature. 2. Allergy: If the finger is exposed to allergenic substances, the finger may also swell, accompanied by pain, itching, reddening and so on. 3. Trauma: When trauma leads to fracture and tendon rupture of a single finger, the typical symptoms are swelling and pain of a single finger, decreased sensitivity of the finger, and limited movement of the finger. 4. Tenosynovitis: It is a kind of aseptic inflammation that occurs in the tendons of the fingers, and its typical manifestations are the swelling and pressure pain of a single finger, as well as the popping sound of the finger when moving. 5. Arthritis: Various types of arthritis can lead to sudden swelling of a single finger, such as osteoarthritis, gouty arthritis and so on. When a single finger suddenly swollen, and accompanied by obvious pain, affecting the hand activities, and even the symptoms of other fingers, should be as soon as possible to seek medical attention, after the cause of the disease, to receive appropriate treatment.