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
What is the etiological agent for buruli ulcer which usually occur in the tropics? 
Select and explain. 
A. Mycobacterium ulcerans, B. Mycobacterium marinum, C. Mycobacterium kansasii, D. Mycobacterium fouitum
A: Mycobacterium ulcerans
Answer: A: Mycobacterium ulcerans 
Explanation: Mycobacterium ulcerans is the causative organism for Buruli ulcer disease. It is the third most frequent mycobacterial pathogen, after M. tuberculosis and M. leprae. Children are most often affected. Infected individuals develop severe ulceration involving the skin and subcutaneous tissue that is often progressive unless treated effectively. Surgical excision and grafting are usually needed. Antimicrobial treatment is often unsuccessful. Ref: Sethi A. (2012). Chapter 184. Tuberculosis and Infections with Atypical Mycobacteria. In L.A. Goldsmith, S.I. Katz, B.A. Gilchrest, A.S. Paller, D.J. Leffell, N.A. Dallas (Eds), Fitzpatrick's Dermatology in General Medicine, 8e.