Final answer:
Based on the patient's presentation, the most likely suspect is Lyme disease, which is commonly associated with a distinctive bulls-eye rash, flu-like symptoms, and joint pain and swelling.
Step-by-step explanation:
Based on the patient's presentation of a small, painful blister on her inner thigh and a rash on her lower extremities and truncal area, combined with the information that she and her family returned from a camping trip 2 days ago, the most likely suspect is Lyme disease. Lyme disease is commonly associated with a distinctive bulls-eye rash, flu-like symptoms, and joint pain and swelling. It is important to note that not all patients with Lyme disease develop the characteristic rash.