While Staphylococcus aureus is a Gram-positive coccus, it is catalase positive, unlike the organism in this case.

Escherichia coli is a Gram-negative rod, not a Gram-positive coccus.

Neisseria gonorrhoeae is a Gram-negative diplococcus. Moreover, it would not typically cause a postpartum infection with uterine tenderness and abnormal lochia.

Streptococcus agalactiae, or Group B streptococcus, is a common cause of postpartum infection. It is a Gram-positive coccus that is catalase negative and beta-hemolytic on blood agar. Postpartum infection with this organism often presents with signs of uterine infection, such as fever, abdominal pain, and abnormal, often foul-smelling lochia.