A 16-year-old boy presents with a 24-hour history of severe abdominal pain, nausea, vomiting, and low-grade fever. The pain is initially periumbilical in location but has migrated to the right lower quadrant of the abdomen, with maximal tenderness elicited at a site one-third of the way between the crest of the ileum and the umbilicus (McBurney point). The leukocyte count is 14,000/mm3, with 74% segmented neutrophils and 12% bands. Surgery is performed. Which of the following describes the expected findings at the affected site?