CT/MRCP shows inflammation of gallbladder, liver and spleen. Sev RUQ pain. 18yo. ANA positive, IgG low, no stones. Dr ordered hida scan. Ideas?

See rheumatologist. You appear to have an autoimmune and/or immune deficiency problem. I note that you are already on Plaquenil (hydroxychloroquine) (SLE?). If you are already under the care of a rheumatologist, consider getting an immune evaluation as well because of the low IgG (how low?). If the problem progresses, you need to go to an university center for further workup.
Biliary disease. You're doctor is looking for evidence of cholecystitis (infection of the gallbladder) or any obstruction in the bile ducts. Cholecystitis can occur without a gallstones which is known as 'acalculus cholecystitis.' Any liver disease or IBD (crown's or UC)?