**Core Concept:** Peritrichous flagella are flagella arranged in a circle around the cell periphery. This arrangement is commonly found in certain bacteria.

**Why the Correct Answer is Right:** The correct answer, **Bacteroides fragilis**, is a gram-negative anaerobic bacterium that belongs to the phylum Firmicutes. It is found in the gastrointestinal tract, particularly in the colon, and is a part of the normal flora. The peritrichous arrangement of flagella on the cell surface allows for efficient swimming and movement in the complex environment of the human gut.

**Why Each Wrong Option is Incorrect:**

A. **Clostridium perfringens (Clostridium perfringens)** is a gram-positive, anaerobic bacterium that belongs to the phylum Firmicutes. It is involved in various diseases and infections, not peritrichous flagella.

B. **Fusobacterium nucleatum (Fusobacterium nucleatum)** is a gram-negative, anaerobic bacterium that belongs to the phylum Firmicutes. Although it is part of the normal gut flora, it does not have peritrichous flagella.

C. **Actinomyces israelii (Actinomyces israelii)** is a gram-positive, anaerobic bacterium that belongs to the phylum Actinobacteria. It is part of the normal oral flora but does not have peritrichous flagella.

D. **Lactobacillus acidophilus (Lactobacillus acidophilus)** is a gram-positive, facultatively anaerobic bacterium that belongs to the phylum Firmicutes. It is a part of the normal gastrointestinal flora but does not have peritrichous flagella.

**Clinical Pearl:** Understanding the differences in bacterial flagella arrangements can help differentiate between normal flora and potential pathogens in clinical microbiology. This knowledge is essential for diagnosing infections and diseases caused by bacteria. For example, distinguishing between peritrichous flagella (e.g., Bacteroides fragilis) and monoklastic flagella (e.g., Streptococcus pyogenes) can help in diagnosing conditions like pyogenic liver abscess caused by Streptococcus pyogenes.