What is the most likely organism responsible for an infection in a child bitten by a dog 3 days prior?

The organism most likely responsible for an infection in a child who has been bitten by a dog is Pasteurella multocida. This bacterium is commonly found in the mouths of dogs and cats and is a frequent cause of infections resulting from animal bites, particularly those inflicted by dogs.

In the case of a dog bite, Pasteurella multocida typically manifests within a day or two after the bite, but infections may occur later. The pathogen is known to cause soft tissue infections, which can include cellulitis and abscess formation. It has characteristics that allow it to thrive in the environment of a bite wound, where there is tissue damage and a microbial imbalance, making it especially virulent in these situations.

Other organisms listed, while they can be involved in different types of infections or wounds, are less specifically associated with dog bites. Streptococcus pyogenes, for instance, is commonly linked to skin infections and throat infections but not as a primary pathogen in dog bite wounds. Escherichia coli is primarily associated with gastrointestinal infections and is less likely to be involved in a bite scenario. Staphylococcus aureus is a common skin inhabitant and can cause infections but is not as characteristic of dog bite wounds as Pasteurella