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: Medicine
What is drug of choice for ulcerative colitis : 
Select and explain. 
A. Salazopyrine, B. Pedinisolone, C. Mercaptopurine, D. 5-amino salicylic acid
D: 5-amino salicylic acid
Answer: D: 5-amino salicylic acid 
Explanation: Answer is D (5-Aminosalicyclic acid) Read text below 'The mainstay of therapy for inflammatory bowel disease remains 5 Aminosalicyclic derrivativs. - CMDT 5 Aminosalicvclic acid: It is used in active treatment of ulcerative colitis and Crohn's disease and during disease inactivity in order to maintain remission. Commonly used formulations of 5 ASA are : Sulfasalazine Oral mesolamine agents Azo compounds : Balsalazide, Olsalazine