Each of the following molars characteristically presents with THREE cusps EXCEPT one. Which is the EXCEPTION?

Correct answer: B. The mandibular second molar

• A. The MAXILLARY SECOND MOLAR frequently shows a markedly reduced distolingual cusp, giving a compressed rhomboidal or heart-shaped outline that functions as a three-cusp form.
• B. CORRECT — this is the exception. The MANDIBULAR SECOND MOLAR has FOUR cusps in the classic arrangement: mesiobuccal, distobuccal, mesiolingual and distolingual, separated by a cross-shaped or 'plus' groove pattern. It does not reduce to three. (The mandibular FIRST molar has five, with a distal cusp and a Y-shaped groove pattern.)
• C. The MAXILLARY FIRST MOLAR has four well-developed cusps plus the cusp of Carabelli as a fifth supplemental cusp — but the question of three-cusp forms concerns the reduction of the distolingual cusp, which in the first molar is well developed.
• D. The MAXILLARY THIRD MOLAR is the most variable tooth in the mouth and characteristically shows a heart-shaped occlusal outline with THREE functional cusps, the distolingual cusp being absent or vestigial.

Note: Distolingual cusp reduction increases as you move posteriorly in the maxilla: first molar (large) → second molar (reduced) → third molar (absent or vestigial). Mandibular molars go the other way on cusp NUMBER: first molar 5 cusps, second molar 4.