Chlamydia are cultured in which of the following?

Chlamydia are obligate intracellular parasites, meaning they require a host cell to replicate and grow. McCoy cells, a type of fibroblast cell line derived from mouse tissue, are specifically used for the culture of Chlamydia because they provide the necessary cellular environment for the bacteria to thrive. This cell culture technique allows for the observation and study of Chlamydia's growth and reproductive cycles, which cannot be achieved through traditional bacteriological media like blood agar.

In contrast, other options mentioned do not provide the specific conditions needed for Chlamydia growth. For example, while the yolk sac of a chick embryo can be used for certain types of viral cultures, it is not typically used for Chlamydia. Blood agar is a general-purpose medium that supports the growth of many bacteria, but it lacks the required cellular components for cultivating obligate intracellular organisms like Chlamydia. Tissue culture as a broader term could encompass McCoy cells, but without specifying a cell line, it does not pinpoint the ideal conditions for Chlamydia culture.