Final answer:
Staphylococcus aureus is the most likely organism to cause purulent cellulitis in a 41-year-old male patient following a dog bite on his left lower leg.
Step-by-step explanation:
The nurse practitioner knows that the most likely organism to cause purulent cellulitis following a dog bite is Staphylococcus aureus. This is because wounds such as dog bites are known for introducing Staphylococcus, Streptococcus, and other bacteria into the skin and underlying tissues. While Pasteurella multocida is commonly associated with animal bites, it typically presents with a more rapid onset of signs and symptoms post-injury. In comparison, Staphylococcus aureus is a common cause of skin infections like cellulitis and can lead to serious complications if left untreated.