A child presents with a unilateral posterior crossbite associated with a functional shift of the mandible. The most common underlying cause is:

Most 'unilateral' posterior crossbites arise from a symmetric but narrow maxilla. The narrow arch forces the child to shift the mandible laterally to reach maximum intercuspation, producing a crossbite that only appears unilateral. Treatment is maxillary expansion.Other options: True mandibular asymmetry is uncommon; missing molars and excess maxillary width do not produce this functional shift.Key exam takeaway: Unilateral posterior crossbite is usually a narrow maxilla plus a functional shift; treat with expansion, ideally early.