What is the most likely pathogen causing cellulitis in a 66-year-old woman with a painful red patch on her leg?

In the context of cellulitis, particularly in adults, Staphylococcus aureus is the primary pathogen responsible for this skin and soft tissue infection. This organism is commonly found on the skin and can enter through breaks or abrasions, leading to infection. In older patients, like the 66-year-old woman in the scenario, the immune response may be less robust, which can predispose them to skin infections like cellulitis.

While other bacteria can cause soft tissue infections, Staphylococcus aureus, including its methicillin-resistant strain (MRSA), is a leading cause in community-acquired cases. The characteristic signs of cellulitis, such as the painful red patch observed in this patient, align with what is typically expected when S. aureus is the causal organism.

Other options, such as Haemophilus influenzae, are typically associated with respiratory infections or conditions in children but are less common in cellulitis among adults. Proteus mirabilis, primarily known for causing urinary tract infections, and Escherichia coli, which is commonly associated with gastrointestinal infections, are also not typical pathogens for cellulitis in healthy adults. Therefore, Staphylococcus aureus stands out as the most plausible pathogen for cellulitis in this case.