In a case of a patient with swollen lymph nodes and gram-negative rods resembling "closed safety pins," what is the most likely organism?

• A. Yersinia pestis
• B. Klebsiella pneumoniae
• C. Proteus vulgaris
• D. Morganella morganii

Answer: (A)

The correct organism in this scenario is Yersinia pestis. This bacterium is known to cause plague and is characterized by its distinctive morphology, particularly when stained and examined under a microscope. The appearance of “closed safety pins” refers to the polar staining observed in Yersinia pestis, which results in a unique bi-polar (or "safety pin") appearance when viewed as Gram-negative rods.

Yersinia pestis’s transmission is primarily through flea bites or contact with infected animals, which correlates with symptoms like swollen lymph nodes, known as buboes, that are commonly seen in plague cases. The combination of the gram-negative rod shape and the characteristic staining pattern strongly suggests the presence of this organism in a clinical context, especially with the accompanying symptomatology.

Other options, while they are indeed members of the Enterobacteriaceae family and can produce infections, do not exhibit the specific morphology or clinical presentation associated with Yersinia pestis. For instance, Klebsiella pneumoniae tends to be larger and encapsulated, Proteus vulgaris is primarily known for its swarming motility on agar, and Morganella morganii is not typically associated with the characteristic lymphadenopathy seen in plague. Thus, the clinical presentation