A 51-year-old man is referred to dermatology by his primary care provider for evaluation of a lesion on his neck. Present for several years, it has waxed and waned in prominence and sensitivity. Occasionally, the lesion drains foul-smelling pus and the entire area becomes red and swollen. Oral antibiotics—cephalexin and amoxicillin/clavulanate—help, but the lesion never quite goes away. The patient is otherwise healthy and is not immunosuppressed.

On examination, a 7-mm friable nodule, vascular in appearance, is seen on the upper right anterolateral neck. It is surrounded by diffuse redness and swelling, and it exhibits increased warmth and moderate tenderness to the touch. In the center of the nodule, a tiny draining sinus is noted. No nodes can be felt in the area. Inside the mouth, there is focal swelling and tenderness on the lateral gum line, directly superior to the lesion.

The most likely explanation for this combination of findings is: