A 14-year-old girl presents to a general outpatient clinic reporting persistent left hip pain. She states the pain has been gradually worsening over the past month, radiating to her left knee, and is exacerbated by ambulation. Her medical history includes a significant recent growth spurt and a body mass index (BMI) in the 98th percentile for her age. On examination, her temperature is 37.00C, blood pressure is 118/72 mmHg, heart rate is 98 beats/min, respiratory rate is 16 breaths/min, and oxygen saturation is 99% on room air. She exhibits an antalgic gait, and active and passive range of motion of her left hip is notably restricted, particularly internal rotation and abduction. A radiograph of her pelvis, including both hips, is performed. Complete blood count and inflammatory markers are pending. Considering her clinical presentation and the typical radiographic findings for this age group, what is the most likely diagnosis and the most appropriate immediate intervention?