Most common drug causing peudomembranous enterocolitis is

C.difficile causes acute colitis with bloody and watery diarrhea and pseudomembranous colitis. C.difficle is the most common cause of healthcare-associated diarrhea in many developed countries following the use of broad-spectrum antibiotics like clindamycin, ampicillin, or fluoroquinolones to which the organism is resistant. Ref: Textbook of microbiology; Ananthanarayan and paniker's; 10th Edition; Pg: 270