______ is a group of rare diseases that cause abnormal growth of the body's connective tissue.
Mixed Connective Tissue Disease (MCTD) Pediatric mixed connective tissue disease (MCTD) is a rare disease in children that has symptoms of arthritis along with features of scleroderma, dermatomyositis and lupus. It is characterized by lab tests showing the presence of specific antibodies to nuclear proteins (specifically, RNP). MCTD is a type of autoimmune disease.