Which of the following is a small, nonmotile, coccobacillus that grows in egg yolk sacs?

The correct choice is Rickettsia, which are small, nonmotile coccobacilli known for their unique growth requirements and characteristics. Rickettsiae are obligate intracellular pathogens, meaning they can only grow and reproduce inside the cells of a host. They are typically cultivated in embryonated eggs, particularly in the yolk sac, allowing for the production of infectious agents while ensuring the maintenance of their specific growth conditions.

The morphology of Rickettsia as coccobacilli is also an important point; they are approximately the size of bacteria that are both spherical and rod-shaped, which fits the description. Their classification and lifestyle, alongside their growth in egg yolk sacs, particularly aligns them with the question's criteria regarding growth environments.

Other bacteria in the choices do have distinct characteristics: Chlamydia is also an obligate intracellular pathogen but typically shows a different cellular morphology and is not specifically mentioned to grow in yolk sacs. Escherichia coli is a motile bacillus commonly found in the intestines and does not require special environments like egg yolk sacs for growth. Bordetella, while nonmotile, is primarily known for its involvement in diseases such as whooping cough and generally grows in different