**Question:** Of the following four bacterial species, which is LEAST LIKELY to be found in plaque?

A. Actinomyces israellii
B. Actinobacillus actinomycetemcomitans
C. Campylobacter rectus
D. Fusobacterium nucleatum

**Correct Answer: D. Fusobacterium nucleatum**

**Core Concept:** Plaque is a biofilm that forms on tooth surfaces and is composed primarily of anaerobic bacteria. It is a precursor to dental caries and periodontal disease.

**Why the Correct Answer is Right:** Fusobacterium nucleatum is an anaerobic bacterium that plays a role in the early stages of plaque formation. However, it is not the least likely bacterium to be found in plaque. The least likely options are Actinomyces israellii and Actinobacillus actinomycetemcomitans.

**Why Each Wrong Option is Incorrect:**

A. Actinomyces israellii (Option A) is a Gram-positive, anaerobic bacterium that is part of the normal oral flora and is commonly found in dental plaque. It is not the least likely option.

B. Actinobacillus actinomycetemcomitans (Option B) is a Gram-negative, facultative anaerobic bacterium that is associated with aggressive periodontitis, a severe form of gum disease. It is not the least likely option to be found in plaque.

C. Campylobacter rectus (Option C) is a Gram-negative, anaerobic bacterium that is a component of the normal oral flora and is commonly found in dental plaque. It is not the least likely option.

D. Fusobacterium nucleatum (Option D) is an anaerobic bacterium that plays a role in the early stages of plaque formation. However, it is not the least likely bacterium to be found in plaque.

**Clinical Pearl:**
Fusobacterium nucleatum is an important player in the early stages of plaque formation, which is why it is not the least likely option. However, Actinomyces israellii and Actinobacillus actinomycetemcomitans are more commonly associated with severe periodontal diseases like aggressive periodontitis, making them the least likely options.

**Why Option D is Incorrect:**

While Fusobacterium nucleatum plays a significant role in the initial colonization of plaque, it is not the least likely bacterium to be found in plaque. Actinomyces israellii and Actinobacillus actinomycetemcomitans are more commonly associated with severe periodontal diseases like aggressive periodontitis, making them the least likely options.

**Why Options A, B, and C are Incorrect:**

A, B, and C are all components of the normal oral flora and are commonly found in dental plaque. Therefore, they are not the least likely options. Actinomyces israellii and Actinobacillus actinomycetemcomitans, on the other hand, are more commonly associated with severe periodontal diseases such as aggressive