Which of the following is a small, nonmotile, coccobacillus that is cultured in the yolk sac of a chick embryo?

The correct choice is Rickettsia, which are indeed small, nonmotile coccobacilli typically cultured in the yolk sac of chick embryos. Rickettsia are obligate intracellular parasites, which means they require a host's cells for growth and reproduction — a characteristic that often necessitates laboratory methods like culture in animal models such as chick embryos. This method leverages the nutrient-rich environment provided by the yolk sac, enabling the bacteria to thrive and be isolated for study.

In contrast, Chlamydia, while also an obligate intracellular pathogen, generally utilizes tissue culture techniques rather than chick embryos for cultivation. Mycoplasma are characterized as wall-less bacteria and do not follow the same culture techniques suitable for Rickettsia, often grown in cell-free media. Bifidobacterium are also a different type of bacteria that do not fit the description provided, as they are not typically cultured in yolk sacs but rather in anaerobic conditions or specific selective media designed for culturing gut flora.

Understanding these distinctions is crucial when studying bacterial culturing methods and the unique life cycles of various pathogens.