A 60-year-old man has had persistent bloody diarrhea, abdominal cramps, and fever for the past week. On physical examination, his temperature is 38.1°C. He has mild diffuse abdominal pain. A stool sample is positive for occult blood. Sigmoidoscopic examination shows mucosal ulceration in the cecum and ascending colon. On colonoscopy, marked mucosal erythema with focal ulceration is seen from the rectum to the ascending colon. The ulcers do not penetrate the muscularis propria. A biopsy is performed, and the microscopic appearance of the specimen is shown in the figure. Which of the following infectious organisms is most likely to produce these findings?

□ (A) Giardia lamblia
□ (B) Entamoeba histolytica
□ (C) Shigella flexneri
□ (D) Salmonella enteritidis 

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