A 16-year-old female long-distance runner develops gradually worsening right groin pain over six weeks. She has recently increased her training volume and reports irregular periods; she is afebrile and walks with a shortened stride. Hip radiographs are normal, but hopping on the right leg is painful and internal rotation is restricted. Magnetic resonance imaging shows a narrow low-signal line across the superior aspect of the femoral neck with surrounding marrow oedema. Which one of the following is the most appropriate immediate management?