What is the most common cause of unilateral maxillary crossbite?

Correct answer: D. True maxillary constriction with unilateral collapsed bite

• A. Mandibular expansion would cause the mandibular teeth to extend BEYOND the maxillary teeth — but it is maxillary constriction (insufficient width) that is the primary cause of unilateral posterior crossbite.
• B. Tongue thrusting is associated with anterior open bite — not typically with unilateral posterior crossbite.
• C. Digit sucking can cause crossbite, but it typically produces a bilateral constricted maxilla (from the labial pressures of the digit) — not the most common cause of unilateral crossbite.
• D. The most common cause of UNILATERAL maxillary posterior crossbite is TRUE MAXILLARY CONSTRICTION — a narrow maxillary arch that causes the maxillary posterior teeth to occlude lingually to the mandibular posterior teeth. The 'unilateral' presentation often involves a functional shift (mandibular midline shift to the crossbite side) due to a premature contact that deflects the mandible laterally, though the underlying cause is bilateral maxillary constriction.