This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
Patient presenting with abdominal pain, diarrhea taking clindamycin for 5 days. Treated with metronidazole symptoms subsided. What is the causative agent - 
Select and explain. 
A. Clostridium difficile, B. Clostridium perfringens, C. Clostridium welchi, D. Clostridium marneffi
A: Clostridium difficile
Answer: A: Clostridium difficile 
Explanation: Ans. is 'a' i.e., Clostridium difficile