A 14-year-old boy with sickle cell trait presents with a one-day history of severe pain in the left thigh and knee, unable to bear weight. XR of the knee is normal; hip ROM is limited by pain; WBC and ESR are normal. What is the next best step?

• A. XR of the left hip
• B. MRI of the knee
• C. Ultrasound of the hip
• D. CT scan of the pelvis

Answer: (A)

Vaso-occlusive changes in the femoral head (avascular necrosis) should be on the radar when an adolescent with sickle cell–related conditions presents with hip pain and limited range of motion. The knee imaging being normal points away from distal joints as the source, and the normal inflammatory markers make infection less likely. The hip is the affected area, so starting with radiographs of the left hip is the right move to look for signs of AVN or other hip pathology. If the hip X-ray is nondiagnostic but suspicion remains high, an MRI of the hip would then be the best next test because MRI is more sensitive for detecting early AVN.