A Study of Ustekinumab in Pediatric Participants (U-POPS) With Juvenile Psoriatic Arthritis or Psoriasis

jPsA is a complex, chronic, progressive, debilitating musculoskeletal disease with significant remaining medical need. There is a need for medications which have a similar efficacy profile and a well-characterized safety profile relative to currently available tumor necrosis factor alpha (TNF alpha) inhibitors for jPsA participants with active disease. STELARA (ustekinumab) is a fully human immunoglobulin G1 kappa monoclonal antibody which binds with high affinity to the p40 subunit common to both interleukin (IL)-12 and IL 23 preventing IL-12/23p40 binding to the IL 12 Rb1 cell surface receptor shared by both cytokines. Through this mechanism of action, ustekinumab effectively neutralizes IL-12 T helper 1- and IL-23 T helper 17-mediated cellular responses. Ustekinumab has been extensively studied in adult participants with psoriasis, psoriatic arthritis (PsA), Crohn's disease, and ulcerative colitis (UC). Additionally, ustekinumab has been studied in children (greater than or equal to \[\>=\] 6 to less than \[\<\] 12 years of age) and adolescents (\>=12 to \<18 years of age) with pediatric psoriasis \[PsO\]. This study consists of 3 visits: screening (Visit 1), interim and final visit (Visits 2 and 3) (Visits should be \>=7 days apart). Key safety assessments include analyses of the incidence and types of adverse events (AEs), serious adverse events (SAEs), reasonably related AEs, and discontinuation of ustekinumab due to an AE, infections, and/or injection site and hypersensitivity reactions. Any newly identified malignancy, case of active tuberculosis (TB), or opportunistic infection will also be assessed. The total duration of the study is up to 20 weeks.