A radiograph shows a radiolucent space between the apical end of a post and the remaining gutta-percha, although the root canal filling itself is well done. What is the MOST likely explanation?

Correct answer: D. The post is too short for the prepared post space

• A. The stem states the root canal filling was well done — the obturation is not the problem.
• B. Insufficient canal preparation would show inadequate taper or an underfilled canal, not a discrete gap beneath a post.
• C. An overly wide post produces excessive dentin removal and root fracture risk, visible as a thin remaining root wall, not an apical void.
• D. CORRECT. When the post space is prepared to a given depth but the post placed is SHORTER than that space, a void remains between the post apex and the gutta-percha. This void is a nidus for bacterial leakage, and the shortened post provides inadequate retention and poor stress distribution. Ideal post length is at least equal to the clinical crown height, or two-thirds of the root length, while preserving 4–5 mm of apical gutta-percha seal.

Note: Post rules: ≥ crown height or 2/3 root length, leave 4–5 mm apical GP, preserve a 1.5–2 mm ferrule, and never remove more than 1/3 of root width.