**Core Concept**
Bacillus anthracis, Yersinia pestis, Francisella tularensis, and Brucella spp. are highly virulent pathogens that have been used as biological agents in microbial terrorism. These agents can cause severe disease in humans, with high mortality rates. The potential for use as a biological weapon is influenced by factors such as ease of production, transmission, and virulence.

**Why the Correct Answer is Right**
The correct answer is **D. Brucella spp.** because while Brucella spp. can cause severe disease, it is relatively difficult to aerosolize and transmit, making it less likely to be used as a biological weapon. Brucella spp. require a vector (such as ticks or fleas) to transmit, which limits their potential for aerosol-based terrorism.

**Why Each Wrong Option is Incorrect**
* **Option A: Bacillus anthracis** is a highly virulent agent that can be easily produced and aerosolized, making it a prime candidate for biological terrorism.
* **Option B: Yersinia pestis** is the causative agent of plague, which has been used in bioterrorism attacks in the past. It can be aerosolized and transmitted through the air, making it a significant threat.
* **Option C: Francisella tularensis** is a highly virulent agent that can be aerosolized and transmitted through the air, making it a potential candidate for biological terrorism.

**Clinical Pearl / High-Yield Fact**
It is essential to note that all biological agents require specialized handling and containment to prevent transmission. Healthcare workers and first responders must be aware of the risks associated with these agents and take appropriate precautions to prevent exposure.

**Correct Answer:** D. Brucella spp.