The panoramic and intraoral views show primary molars standing well below the plane of occlusion. What is the cause of this posterior open bite?

Correct answer: C. Ankylosis of the primary molars

• A. Missing successors can delay exfoliation of a primary molar, but retention alone does not drop the tooth below the occlusal plane — ankylosis does.
• B. A thumb habit produces an anterior open bite, not a posterior one.
• C. Correct. An ankylosed primary molar is fused to bone and cannot follow the vertical growth of the alveolus, so the adjacent teeth erupt past it and it appears submerged — infraocclusion. A solid, high-pitched percussion note and loss of the PDL space confirm it.
• D. Primary failure of eruption affects permanent teeth that never erupt despite a clear path; here the primary teeth are present but submerged.