The disease Hansen's disease is caused by which organism?

• A. Mycobacterium kansasii
• B. Mycobacterium avium
• C. Mycobacterium tuberculosis
• D. Mycobacterium leprae

Answer: (D)

Hansen's disease, or leprosy, is caused by Mycobacterium leprae. This organism is a slow-growing acid-fast bacillus that preferentially infects cooler areas of the body such as skin and peripheral nerves, which explains the typical skin lesions and nerve damage seen in the disease. It is the classic etiologic agent specifically associated with leprosy, unlike the other bacteria listed. The other species cause different mycobacterial infections: some produce pulmonary or disseminated disease in various hosts, while Mycobacterium tuberculosis causes tuberculosis. This specific association with M. leprae is what makes it the correct answer.