A child has a unilateral posterior crossbite with a functional shift of the mandible on closure. The most likely skeletal etiology is:

Correct answer: B. Unilateral maxillary constriction

• A. Incorrect. Bilateral maxillary constriction would produce a BILATERAL crossbite, not a unilateral one.
• B. CORRECT. Unilateral posterior crossbite with a mandibular shift on closure is the classic presentation of unilateral maxillary constriction. The maxillary arch is narrow on one side, causing the mandible to deviate laterally to achieve maximum intercuspation. Rapid palatal expansion corrects the maxillary constriction and eliminates the shift.
• C. Incorrect. True skeletal mandibular asymmetry presents as a consistent deviation at all positions — there is NO functional shift, because the deviation is structural, not an accommodation.
• D. Incorrect. Mandibular constriction is uncommon and would typically produce a different clinical picture.