The etiologic agent of Hansen disease (leprosy) is:

• A. M. leprae
• B. M. bovis
• C. M. fortuitum
• D. M. tuberculosis

Answer: (A)

The main concept here is identifying the organism that causes Hansen disease. Hansen disease is caused by Mycobacterium leprae, an acid-fast bacillus that grows extremely slowly and is notoriously difficult to culture on standard laboratory media. It preferentially affects cooler parts of the body, like skin and peripheral nerves, leading to the characteristic lesions and neuropathy seen in leprosy. In research and some diagnostic contexts, M. leprae is grown in the footpads of mice or in armadillos, reflecting its unique growth requirements.

The other organisms listed cause different diseases: M. tuberculosis causes tuberculosis; M. bovis can cause TB-like disease in humans (originating from cattle); M. fortuitum is a rapid-growing non-tuberculous mycobacterium that can cause skin and soft tissue infections but not Hansen disease.