A child has a 2-week history of spiking fevers, which have been as high as 40degC (104degF). She has spindle-shaped swelling of finger joints and complains of upper sternal pain. When she has fever, the parents note a faint salmon-colored rash that resolves with the resolution of the fever. She has had no conjunctivitis or mucositis, but her heart sounds are muffled and she has increased pulsus paradoxus. Which of the following is the most likely diagnosis?

Juvenile idiopathic arthritis (JIA, formerly known as juvenile rheumatoid arthritis) frequently causes spindle-shaped swelling of finger joints and can involve unusual joints such as the sternoclavicular joint. Presentation of JIA occurs as either polyarthritis (five or more joints, systemic symptoms not so severe or persistent), pauciarticular (four or fewer joints, lower-extremity joints, extra-articular disease unusual), or systemic disease (severe constitutional disease, systemic symptoms prior to arthritis, rheumatoid rash, high spiking fevers, variable joint involvement, also known as Still's disease). This disorder can be associated with spiking high fevers and diffuse rash, which are not a feature of rheumatic fever, toxic synovitis, or osteoarthritis. Although septic arthritis can affect any joint, it would not be likely to affect finger joints by causing spindle-shaped swellings. Toxic synovitis usually involves larger joints, such as the hip, and osteoarthritis is not a disease of childhood.