**Core Concept**
**Flagella Structure and Motility in Bacteria**
Bacteria exhibit diverse flagellar structures, influencing their motility and survival in different environments. The presence of sheathed flagella is a characteristic feature of certain bacterial species, impacting their virulence and pathogenicity.

**Why the Correct Answer is Right**
The bacterium *Helicobacter pylori* (H. pylori) is known for its unique flagellar structure, consisting of seven sheathed flagella. This distinctive feature enables H. pylori to exhibit strong motility, facilitating its colonization of the gastric mucosa and contributing to its pathogenicity. The sheathed flagella of H. pylori are composed of a protein structure that provides stability and enables the bacteria to move efficiently through the mucus layer of the stomach. This unique flagellar structure is a key factor in the bacterium's ability to cause gastric ulcers and other gastrointestinal disorders.

**Why Each Wrong Option is Incorrect**
**Option A:** *Vibrio cholerae* has a single polar flagellum, which provides motility but does not consist of seven sheathed flagella.
**Option C:** *Pseudomonas aeruginosa* exhibits polar and peritrichous flagella, but its flagellar structure is not characterized by seven sheathed flagella.
**Option D:** Spirochetes, such as *Treponema pallidum*, have a unique spiral shape and exhibit endoflagella, but they do not possess seven sheathed flagella.

**Clinical Pearl / High-Yield Fact**
The presence of seven sheathed flagella in H. pylori is a key feature of its pathogenicity, enabling it to colonize the gastric mucosa and cause gastrointestinal disorders. This characteristic flagellar structure is a critical factor in the bacterium's ability to evade the host's immune response and persist in the stomach.

✓ Correct Answer: B. H. pylori