Avascular Necrosis Femoral Head

A 42-year-old man presents with a 6-month history of progressive left groin pain and restricted internal rotation of the hip. He has been on high-dose prednisolone (25 mg daily) for 3 months for systemic lupus erythematosus. MRI of the left hip is performed. The structure marked **A** in the diagram shows a pathognomonic finding on T2-weighted imaging. What is this radiological sign, and what does it indicate about the underlying pathophysiology of this patient's condition?