This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Inflammatory Bowel Disease, Subject name: Medicine
Treatment of choice in ulcerative colitis is: 
Select and explain. 
A. 5 - aminosalicylic acid, B. Azathiorprine, C. Metronidazole, D. Salicylates
A: 5 - aminosalicylic acid
Answer: A: 5 - aminosalicylic acid 
Explanation: 5-ASA is the active metabolite of sulfasalazine which exes anti-inflammatory action. The mainstay of therapy for mild to moderate UC is sulfasalazine and the other 5-ASA agents. These agents are effective at inducing and maintaining remission in UC. They may have a limited role in inducing remission in CD but no clear role in maintenance of CD.