Tooth #30 (mandibular right first molar) has been endodontically treated and now requires a post and core. Which root should be selected for the post space?

Correct answer: B. Distal

• A. The roots are not equivalent. Root morphology dictates post site selection, and choosing the wrong root converts a restorable tooth into a perforated one.
• B. CORRECT. In a MANDIBULAR MOLAR the DISTAL root is the root of choice for a post. It is the largest root, its canal is the widest and roundest, and it is the straightest — the mesial root is thin, ribbon-shaped in cross-section, curved, and has a pronounced mesial concavity (the 'danger zone') on its distal surface where the dentin is thinnest. Post preparation in a mesial canal readily causes strip perforation.
• C. The MESIOLINGUAL canal shares the same problems as the mesiobuccal — it sits within the same thin, hourglass-shaped mesial root with minimal dentin on its distal wall.
• D. The MESIOBUCCAL canal is narrow, curved, and lies immediately adjacent to the furcal concavity. Preparing a post space here is the classic route to a strip perforation into the furcation.

Note: The parallel rule for the MAXILLARY molar is the PALATAL root — largest, roundest, straightest. Same logic, different tooth. And remember the general post rules that apply regardless of root: preserve 4–5 mm of apical gutta-percha, keep at least 1 mm of circumferential dentin around the post, and secure a 1.5–2 mm ferrule of sound coronal dentin, which is the strongest single predictor of long-term survival.