What is the most likely source of infection for a pregnant woman with fever and gram-positive rods showing tumbling motility in blood culture?

The most likely source of infection in a pregnant woman with fever and gram-positive rods showing tumbling motility in blood culture is fresh, raw cow's milk. This is associated with Listeria monocytogenes, a bacterium that is particularly notable for its characteristic tumbling motility when observed under a microscope, especially at room temperature.

Listeria is often found in unpasteurized dairy products, including raw milk, and can pose significant health risks during pregnancy, leading to serious complications such as listeriosis, which can affect both the mother and the fetus. The infection can manifest with flu-like symptoms, including fever, and can progress to more severe symptoms if not treated.

Other options like well-done roast beef, homemade applesauce, and baked apple pie are less likely sources for this particular pathogen. While Listeria can also be found in processed meats and dairy products, raw milk is the most commonly associated source of infection for pregnant women. Thus, fresh, raw cow's milk is the most appropriate choice for this scenario.