**Core Concept**
Query fever, also known as tularemia, is a zoonotic infection caused by the bacterium Francisella tularensis. This bacterium is typically transmitted to humans through contact with infected animals, contaminated water, or vectors like ticks and deer flies.

**Why the Correct Answer is Right**
The correct answer is **Francisella tularensis**, a highly infectious and virulent bacterium that can cause a range of symptoms, including fever, chills, and swollen lymph nodes. **Francisella tularensis** has a unique ability to survive within macrophages, allowing it to evade the host's immune system and cause persistent infection. The bacterium's small size and high virulence make it a significant public health concern.

**Why Each Wrong Option is Incorrect**
**Option A:** Brucella abortus is a different bacterium that causes brucellosis, a zoonotic infection typically transmitted through contact with infected animals or their products. While both infections can cause fever, they are distinct diseases with different causative agents and clinical presentations.
**Option B:** Coxiella burnetii is the causative agent of Q fever, another zoonotic infection that can cause fever, fatigue, and pneumonia. However, it is not responsible for query fever.
**Option C:** Yersinia pestis is the bacterium that causes plague, a highly infectious disease that can cause fever, chills, and swollen lymph nodes. However, it is not the causative agent of query fever.

**Clinical Pearl / High-Yield Fact**
To remember the different forms of tularemia, recall the acronym "F.T.S." - Francisella tularensis can cause **F**ever, **T**ularemia, and **S**wollen lymph nodes.

**Correct Answer:** A. Francisella tularensis