A 30-year-old Florida male comes to a physician for evaluation of what he thought might be brown recluse spider bite. He recalls no contact with spiders but was working under the house two days earlier. On exam, he is febrile (38.3°C), but not toxic-appearing. The lesion in question is on the abdomen and it is rounded, 3 cm in diameter, red, tender, and fluctuant.

What is the most likely cause?