A 68-year-old man from Delhi presents with a 3-month history of progressive left hip pain and stiffness. He reports difficulty walking and cannot squat or cross his legs. On examination, the left hip is held in flexion, adduction, and internal rotation. Passive abduction and external rotation are severely restricted and painful. Plain radiograph of the pelvis shows joint space narrowing and osteophytes around the left hip. Which anatomical structure is primarily responsible for the restricted external rotation observed in this patient?