Which organism causes plague?

• A. Yersinia pestis
• B. Bacillus anthracis
• C. Francisella tularensis
• D. Brucella abortus

Answer: (A)

Plague is caused by Yersinia pestis, a Gram-negative bacterium that cycles between fleas and rodent hosts. Humans typically become infected through bites from infected fleas or contact with infected animal tissues; inhalation can lead to pneumonic plague, which spreads rapidly between people. This organism is the classic cause of plague and is specifically associated with that disease pattern, including buboes in the lymph nodes and potential progression to severe septicemia or pneumonia if not treated.

The other organisms listed produce different diseases: Bacillus anthracis causes anthrax, Francisella tularensis causes tularemia, and Brucella abortus causes brucellosis. Their transmission routes, reservoirs, and clinical presentations differ from plague, which is why the distinguishing agent for plague is Yersinia pestis.