A 14-year-old girl has a primary second molar (tooth K) that is markedly infraoccluded relative to the adjacent teeth. What is the MOST likely cause?

Correct answer: B. Ankylosis of the primary molar

• A. Hypophosphatasia causes PREMATURE loss of primary teeth with intact roots due to cementum defects — the opposite of retention.
• B. CORRECT. ANKYLOSIS is fusion of cementum/dentin directly to alveolar bone with loss of the intervening PDL. The ankylosed tooth cannot follow normal vertical alveolar growth, so as the adjacent teeth continue to erupt it appears progressively submerged (infraoccluded). Clinically it gives a high-pitched, solid percussion note; radiographically the PDL space is absent in at least one area. Primary MANDIBULAR molars are most commonly affected. Management depends on the presence of a successor and the severity of the infraocclusion.
• C. Hypothyroidism delays eruption generally and causes macroglossia and delayed exfoliation, but it does not selectively submerge a single tooth.
• D. An impacted successor may accompany ankylosis, but the infraocclusion itself is caused by the ankylosis, not the impaction.

Note: Percussion is your chairside test: a solid, high-pitched 'clink' instead of a cushioned note means ankylosis.