## **Core Concept**
The question tests knowledge of a specific type of organism characterized by its flagellation and its ability to cause pneumonia. Unipolar flagellate organisms are typically bacteria. The key here is to identify which one is known to cause pneumonia.

## **Why the Correct Answer is Right**
The correct answer, **Legionella pneumophila**, is a gram-negative bacterium known for causing Legionnaires' disease, a type of pneumonia. It is characterized by having a single polar flagellum, making it a unipolar flagellate organism. Legionella species are often found in water systems and can cause outbreaks of pneumonia.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Streptococcus pneumoniae* is a gram-positive coccus, not a unipolar flagellate organism, and it causes pneumonia but through a different mechanism and characteristics.
- **Option B:** *Mycoplasma pneumoniae* is known for causing pneumonia but it does not have flagella; it's a cell wall-less bacterium.
- **Option D:** *Chlamydophila pneumoniae* causes pneumonia but it is an obligate intracellular pathogen and does not have flagella.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Legionnaires' disease, caused by *Legionella pneumophila*, often presents with severe pneumonia, gastrointestinal symptoms, and can be associated with outbreaks related to contaminated water systems. A useful mnemonic is not directly applicable but knowing that Legionnaires' disease can be severe and requires specific antibiotic treatment (like erythromycin or azithromycin) is crucial.

## **Correct Answer:** C. Legionella pneumophila.