Which organism is the causative agent of Hansen disease?

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

Answer: (A)

Hansen disease is caused by a slow-growing, acid-fast bacillus that specifically targets skin and peripheral nerves, leading to the characteristic nerve damage and skin lesions. This organism is Mycobacterium leprae, the classic etiologic agent of leprosy. It is distinguished from other mycobacteria by its very long incubation, poor ability to be cultured in standard artificial media, and its preference for cooler body temperatures, which helps explain why it primarily affects skin and nerves in the extremities. In practice, diagnosis often relies on clinical signs plus detection of acid-fast bacilli in skin or nerve tissue.

The other organisms listed cause different mycobacterial diseases with distinct clinical patterns—for example, Mycobacterium tuberculosis causes tuberculosis, while Mycobacterium kansasii and Mycobacterium avium complex typically lead to pulmonary disease or disseminated infections, not Hansen disease.