On a maxillary first molar, which furcation entrance lies CLOSEST to the cementoenamel junction?

Correct answer: A. The mesial furcation

• A. CORRECT. The MESIAL furcation of the maxillary first molar has the SHORTEST root trunk — roughly 3 mm from the CEJ, compared with about 4 mm buccally and 5 mm distally. Two consequences follow. First, the mesial furcation is the one most readily involved as attachment is lost, so it is often the earliest furcation defect to appear. Second, it is entered from the PALATAL aspect, not the buccal, because it sits toward the palatal half of the mesial surface — a point routinely missed on probing.
• B. The buccal furcation entrance sits at an intermediate distance from the CEJ, approximately 4 mm, and is the most accessible to probe.
• C. The distal furcation has the LONGEST root trunk, approximately 5 mm, so it is the last of the three to become involved.
• D. The three differ measurably. Shortest root trunk mesially, longest distally — this asymmetry drives both the diagnosis and the prognosis.

Note: Probe all THREE furcations on every maxillary molar, and approach the mesial one from the PALATAL. A Nabers probe used only buccally will systematically miss mesial and distal involvement.