RecruitingMedical Follow-up of New Cases of Polyarthritis in Children and Young AdultsPopulation: Juvenile idiopathic arthritis (JIA), rheumatoid arthritis (RA) and seronegative / psoriatic / undifferentiated arthritis (UA), systemic lupus erythematosus (SLE) or diffuse systemic sclerosis dSS). Naïve to basic treatment OR treated for ≤ 3 months; except for patients with JIA. These 5 cohorts will be subject to standardized clinical monitoring.