Etiology of plague — Which organism is the causative agent of plague in humans?

Introduction / Context:
Plague is a severe zoonotic infection historically responsible for pandemics. Correctly identifying the causative organism is fundamental for diagnosis, public health reporting, and outbreak control.

Given Data / Assumptions:

• The question asks specifically for agents that can cause plague.
• Only one of the listed organisms is known to cause plague.
• We assume standard clinical microbiology knowledge.

Concept / Approach:
Yersinia pestis is the etiologic agent of plague (bubonic, septicemic, and pneumonic forms). It is transmitted primarily via flea vectors from rodent reservoirs or via aerosols in pneumonic plague. Proteus mirabilis and Pseudomonas aeruginosa are important pathogens in other contexts (e.g., urinary infections and opportunistic healthcare-associated infections) but do not cause plague.

Step-by-Step Solution:

Verification / Alternative check:
Laboratory identification includes bipolar (“safety-pin”) staining, growth characteristics, and detection of F1 antigen; epidemiology aligns with flea-rodent cycles.

Why Other Options Are Wrong:

• Proteus mirabilis: uropathogen with swarming motility; not a plague agent.
• Pseudomonas aeruginosa: opportunistic pathogen; unrelated to plague.
• All of these: incorrect because only Y. pestis causes plague.

Common Pitfalls:
Confusing “Yersinia” species; plague is specifically due to Y. pestis, not to other Enterobacterales.

Final Answer:
Yersinia pestis