What is the medical term for a uterus filled with pus?

The medical term for a uterus filled with pus is "pyometra." This condition typically occurs as a result of a bacterial infection and is characterized by the accumulation of pus within the uterine cavity. Pyometra is most commonly seen in unspayed female dogs and cats, particularly in those that have had hormonal influences, such as during or after their heat cycles. The presence of pus indicates an inflammatory response, often associated with bacterial growth in the uterus, resulting from hormonal changes that may predispose the uterus to infection.

In contrast, "cystitis" refers specifically to inflammation of the bladder, while "endometritis" is the inflammation of the inner lining of the uterus, which doesn’t necessarily imply the presence of pus. "Metritis," on the other hand, indicates inflammation of the entire uterine wall but does not specify that it is filled with pus. Therefore, the specificity of "pyometra" in describing a pus-filled uterus makes it the correct answer.