A 32-year-old woman from Delhi presents with acute left hip pain and inability to bear weight following a motor vehicle accident. On examination, the left hip is held in flexion, adduction, and internal rotation; the left leg appears shortened. There is severe pain on any attempt at passive hip movement. Neurovascular examination reveals intact distal pulses and normal sensation. Which structure is most likely injured, and what is the anatomical basis for the characteristic posture?