A patient is hospitalized after an automobile accident. The wounds become infected, and the patient is treated with tobramycin, carbenicillin, and clindamycin. Five days after antibiotic therapy was initiated, the patient develops severe diarrhea and pseudomembranous enterocolitis. Antibiotic-associated diarrhea and the more serious pseudomembranous enterocolitis can be caused by which of the following organisms?