A 9 year old male child presented with history of spiking fevers from last 2 months, which have been as high as 104degF. On examination, he has spindle-shaped swelling of multiple finger joints and complains of upper sternal pain. When he had fever, his mother noticed salmon-colored rash that resolves with the resolution of the fever. He had no conjunctivitis or any oral ulcers, but his hea sounds are muffled and he has pulsus paradoxus. Which of the following is the most likely diagnosis?