Woman returns from Mexico after consuming unpasteurized cheese; most likely etiologic agent?

• A. Bordetella pertussis
• B. Listeria monocytogenes
• C. Staphylococcus aureus
• D. Yersinia enterocolitica

Answer: (B)

Unpasteurized dairy products, especially soft cheeses, are a classic source of Listeria monocytogenes. This organism can grow at refrigerator temperatures, so contaminated cheese from anywhere, including Mexico, can harbor it even when kept cold. Listeria infection often presents as mild gastroenteritis in healthy adults, but it poses serious risk to pregnant women, the elderly, and the immunocompromised, as it can cause fetal loss, neonatal infection, or invasive disease like meningitis. That combination of a soft cheese exposure and the typical high-risk context makes Listeria monocytogenes the most likely culprit in this scenario.