An 8-year-old girl is brought to an urgent care clinic by her parents due to a sudden onset of left hip pain over the last 24 hours. The pain, described as dull and aching, began shortly after she recovered from a mild viral illness characterized by rhinorrhea and a low-grade fever, which her younger brother also had. She is now reluctant to bear weight fully on the left leg, preferring to hop on her right leg. Her past medical history is notable for mild asthma, well-controlled with inhalers. On examination, her vital signs are stable: temperature 37.0°C, heart rate 85/min, blood pressure 95/50 mmHg, respiratory rate 18/min, and oxygen saturation 99% on room air. She appears comfortable at rest but grimaces when asked to move her left hip. There is no visible erythema, swelling, or warmth around the joint. Hip range of motion is mildly restricted with pain elicited on extreme internal rotation and abduction. The girl is able to walk but exhibits a noticeable antalgic gait. Considering her clinical presentation and initial laboratory findings, what is the most likely diagnosis and the initial imaging study indicated to guide her management?