PLE-3568

A 37-year-old man comes to the physician because of a 3-day history of diarrhea. He has had six to eight watery stools daily that occasionally contain streaks of blood. His wife and children do not have similar symptoms. He has not recently traveled. Two weeks ago, he completed a course of oral ciprofloxacin for acute prostatitis. He currently takes no medications. Vital signs are within the normal range. Results of CBC and serum studies are within the reference range. The test for occult blood in the stool is positive. He was started on Metronidazole. What microorganism was the physician entertaining?