An 8-year-old girl after a case of acute respiratory infection has been complaining for the last 2 weeks of a pain in the area of her heart and palpitations. Objectively, her body temperature is febrile, her skin is pale, the heart borders are extended to the left, heart rate is 142/min. The girl presents with paired atrial extrasystoles. dull heart sounds, and a low intensity systolic murmur that reaches its maximum at point 5. Make the provisional diagnosis: