A 5-year-old boy has had diarrhea for a week, averaging six low volume stools per day, which appear mucoid and sometimes blood-tinged. On physical examination, his temperature is 37.4deg C. He has mild lower abdominal tenderness, but no masses. A stool culture is positive for Shigella sonnei. Which of the following microscopic findings would most likely be seen in this child's colon?