A 10-year-old boy is brought to his pediatrician by his parents with concerns about recent onset joint pain and a new rash. They recall he had a sore throat about three weeks ago that resolved spontaneously without medical attention. The current pain initially affected his left knee, then moved to his right ankle, and now involves his left elbow and wrist, described as moderate discomfort. He reports no significant limitation in daily activities but feels generally unwell. There are no reports of significant morning stiffness, persistent fever, chest pain, or dysuria. His past medical history is unremarkable, and he is not on any regular medications. On examination, his temperature is 37.8°C, blood pressure is 105/60 mmHg, pulse is 95/min, and respirations are 18/min. Physical examination reveals mild warmth and tenderness with slight effusion in the left elbow and wrist, though he maintains full range of motion. A faint, non-pruritic, serpiginous rash with central clearing is observed across his trunk. Auscultation of the heart reveals a new grade II/VI holosystolic murmur, most prominent at the apex, radiating to the axilla. Given this presentation, what is the most likely diagnosis and the appropriate initial therapeutic intervention?