What else other than rheumatoid arthritis could cause one extremely painful finger and three other stiff fingers in the morning. Only one hand is affected. Been tested for ra.?

Osteo Arthritis or.. Osteo arthritis or "flexor tendon stenosing tenosynovitis". That's doctor talk for trigger finger. An examination and x-ray can tell which you may have. Over the counter nsaids like Ibuprofen or naproxyn with tylenol (acetaminophen) can help.
Trigger finger(s) Trigger fingers and osteoarthritis commonly present with morning stiffness. Unlike arthritis, trigger fingers become stuck in flexion and may snap with movement. Triggering is due to inflammation of the tendon and tendon sheath. Oral/injectable steroids may resolve symptoms and therapy can be helpful. Sometimes surgical release of the tendon sheath is needed. See an orthopaedic hand specialist.