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
A woman of 45, a known cause of pemphigus vulgaris on a regular treatment with controlled primary disease presented with pain in the right hip and knee. Examination revealed no limb length discrepancy but the patient has tenderness in the scarpa's triangle and limitation of abduction and internal rotation of the right hip joints as compared to the other side. The most probable diagnosis is - 
Select and explain. 
A. Stress fracture of neck of femur, B. Avascular necrosis of femoral head, C. Perthe's disease, D. Transient synovitis of hip
B: Avascular necrosis of femoral head
Answer: B: Avascular necrosis of femoral head 
Explanation: This patient has


Pemphigus vulgaris taking treatment (steroid is the DOC).
Pain in hip and knee (Pain of hip can refer to hip).
Limitation of abduction & internal rotation.


Diagnosis is AVN of hip.