If there is a radiolucent area lateral to a vital tooth, what is the likely diagnosis?

The diagnosis of a lateral periodontal cyst is consistent with the presence of a radiolucent area lateral to a vital tooth. This type of cyst is often found in the interradicular area, adjacent to the roots of vital teeth, particularly in the mandibular premolar area. The lateral periodontal cyst typically does not affect the vitality of the adjacent tooth, which corresponds with the scenario presented.

In contrast, other conditions like the radicular cyst and periapical granuloma are typically associated with non-vital teeth, as they develop as a result of pulpal necrosis or infection. The dentigerous cyst, on the other hand, is usually associated with an impacted tooth and would not generally present as a lateral radiolucent area beside a vital tooth. Thus, the unique characteristics of the lateral periodontal cyst align it distinctly with the signs mentioned in the question.