Kaplan FS, et al. – The clinicians aimed to assess individuals with classic fibrodysplasia ossificans progressiva (FOP) who presented with acute unilateral hip pain to better understand the nature of hip flare–ups in FOP. In FOP, developmental joint pathology could confound the clinical evaluation of hip pain. For suspecting an ossification–prone flare–up of the hip, the most useful modality was lack of sustained response to a brief course of oral prednisone. Evaluation of soft tissue edema by ultrasound or magnetic resonance imaging demonstrated promise in identifying ossification–prone flare–ups and warranted additional analysis in prospective studies.