Explanation
Langenskiöld Stage III infantile Blount disease (tibia vara) in a child over age 3 requires surgical realignment via a proximal tibial valgus and derotational osteotomy.

Clinical Context: Infantile Blount disease is characterized by an abrupt varus angulation below the proximal tibial physis, resulting from disordered endochondral ossification of the medial aspect of the physis (Heuter-Volkmann principle). The Langenskiöld classification describes the progressive radiographic changes. Stage III is marked by a deep 'step-off' or 'cleft' in the medial metaphysis. Bracing (KAFO) is only effective for Stages I and II in children under 3 years of age. Once a child reaches Stage III or age 4, the pathological physeal changes render bracing ineffective, and surgical intervention (proximal tibial osteotomy with overcorrection into 5-10 degrees of valgus) is required to unload the medial physis and prevent permanent physeal arrest. Guided growth is generally reserved for older children (late-onset) without severe physeal depression.

References:
1. Birch JG. 'Blount disease.' J Am Acad Orthop Surg. 2013.
2. Langenskiöld A. 'Tibia vara: osteochondrosis deformans tibiae.' Acta Chir Scand. 1952.