**Core Concept**
Category B bioterrorism agents are moderately easy to disseminate, result in moderate morbidity rates and low mortality rates, and require specific enhancements for diagnostic capacity and enhanced disease surveillance. These agents include bacteria, viruses, and toxins that can cause disease in humans, but are typically not as deadly as Category A agents.

**Why the Correct Answer is Right**
Plague, caused by Yersinia pestis, is a Category A bioterrorism agent due to its high mortality rate and potential for widespread dissemination. It is highly contagious and can cause severe illness, including pneumonic plague, which can spread from person to person. In contrast, Category B agents are generally less deadly and less contagious, making plague a poor fit for this category.

**Why Each Wrong Option is Incorrect**
**Option B:** Ricinus communis, the source of ricin toxin, is a Category B bioterrorism agent due to its potential for causing severe illness and death, as well as its ease of production and dissemination. Ricin toxin can cause severe respiratory and gastrointestinal symptoms, and is highly toxic.
**Option C:** Brucellosis, caused by Brucella bacteria, is a Category B bioterrorism agent due to its potential to cause severe illness and its ease of dissemination. Brucellosis can cause fever, fatigue, and swollen lymph nodes, and can be transmitted through contact with infected animals or contaminated products.
**Option D:** Q fever, caused by Coxiella burnetii, is a Category B bioterrorism agent due to its potential to cause severe illness and its ease of dissemination. Q fever can cause fever, headache, and fatigue, and can be transmitted through contact with infected animals or contaminated products.

**Clinical Pearl / High-Yield Fact**
When considering bioterrorism agents, remember that Category A agents are typically highly contagious and deadly, while Category B agents are less deadly but still pose a significant threat due to their ease of dissemination and potential for widespread illness.

**✓ Correct Answer: A. plague**