This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Orthopaedics
Avascular necrosis of the hip, investigation of choice - 
Select and explain. 
A. MRI, B. USG, C. CT, D. XRAY
A: MRI
Answer: A: MRI 
Explanation: It is a case of avascular necrosis of the femoral head which is one of the late complications of posterior dislocation of the hip.
If the dislocation is reduced early, avascular necrosis develops in at least 10% of the cases of dislocation.
If the reduction is delayed for more then a few hours (as with this patient) the figure rise to 40%.
Pain is a common presentation of avascular necrosis of head.
X-ray of the patient is normal because in the early stages bones are usually normal.
The first X-ray signs probably appear only 6-9 months after the occurrence of bone death.
MRI shows characteristic changes in the marrow long before the appearance of X-ray signs.
Therefore MRI is the most reliable way of diagnosing marrow changes and bone ischaemia (Avascular necrosis) at a comparatively early stage.