4. Gout or Pseudogout
If the swelling comes on suddenly and is very painful, the cause may be gout—a type of arthritis caused by uric acid crystals. Pseudogout, which involves calcium crystals, can cause similar symptoms. These conditions often affect only one joint at a time and may return periodically.
5. Osteoarthritis (Localized)
Though osteoarthritis typically affects both knees over time, it can start in just one knee, especially if that knee has experienced past injuries or bears more weight. Swelling may be mild to moderate and worsen with activity.
