**Core Concept**
A capsulated organism is a type of bacterium that has a thick, protective layer of polysaccharide material surrounding its cell wall. This capsule, also known as a slime layer, plays a crucial role in the pathogenesis of the organism by providing resistance to phagocytosis, desiccation, and other environmental stresses.

**Why the Correct Answer is Right**
The capsule is composed of repeating units of polysaccharide molecules that are covalently linked together. The capsule is produced through the action of enzymes called polysaccharide synthases, which use nucleotide sugar precursors to synthesize the polysaccharide chains. The capsule is essential for the virulence of encapsulated bacteria, as it allows them to evade the host's immune system and cause disease. In the case of *Streptococcus pneumoniae*, the capsule is composed of a specific type of polysaccharide called pneumococcal polysaccharide, which is responsible for the organism's ability to cause pneumonia and other infections.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specifically refer to a type of bacterial capsule. While some bacteria may have a capsule-like structure, not all of them are encapsulated organisms.
* **Option B:** This option is incorrect because it refers to a type of bacterial flagellum, which is a different structure from a bacterial capsule.
* **Option C:** This option is incorrect because it refers to a type of bacterial endospore, which is a highly resistant, dormant structure that some bacteria can form in response to environmental stress.

**Clinical Pearl / High-Yield Fact**
The polysaccharide capsule of encapsulated bacteria is a critical virulence factor that allows them to cause disease. Vaccines against encapsulated bacteria, such as pneumococcal conjugate vaccine, target the polysaccharide capsule to prevent infection.

**Correct Answer:** D.