**Core Concept**
The periapical abscess is a type of **dental infection** that occurs at the apex of a tooth, typically as a result of **bacterial invasion** of the pulp. The most common organisms involved are **anaerobic bacteria**.

**Why the Correct Answer is Right**
Given the context, the most common organism involved is likely to be an anaerobic bacterium, with **Fusobacterium** and **Peptostreptococcus** being examples, but the question is incomplete for a specific answer. However, in general, **Streptococcus** species, particularly **Streptococcus milleri**, are often implicated in such infections due to their ability to form abscesses.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option provided, it's challenging to determine its accuracy.
**Option B:** Similarly, without the specific details, it's hard to assess its correctness.
**Option C:** and **Option D:** also lack specific information to evaluate their validity.

**Clinical Pearl / High-Yield Fact**
In clinical practice, it's essential to remember that periapical abscesses are **polymicrobial infections**, often involving a mix of aerobic and anaerobic bacteria.

**Correct Answer:** D. Streptococcus milleri.