Perthes disease — branching viva

Outpatient clinic: a six-year-old boy with a three-week limp that comes and goes, worse after activity, who points to his right knee when asked where it hurts, and whose hip loses abduction and internal rotation. The examiner asks: describe your diagnostic approach, give your likely diagnosis and its pathophysiological basis, and outline the management — then branches to the Herring lateral pillar classification, the containment decision by age and pillar grade, the older child at onset, and finally to a nine-year-old girl with a collapsing lateral pillar C head that reframes the prognosis and the urgency.