Which population is most likely to present with MAC (Mycobacterium avium complex) disease?

• A. Immunosuppressed individuals or those with underlying lung structural changes
• B. Healthy young adults
• C. Children with asthma
• D. Elderly individuals with COPD only

Answer: (A)

MAC disease is most likely in people whose immune defenses against intracellular bacteria are weakened or whose lungs have structural abnormalities. In practice this means immunosuppressed individuals (for example, advanced HIV infection, organ transplant recipients, or those on long-term steroids) or people with underlying lung changes such as bronchiectasis or COPD with cavities. In these hosts the MAC organisms can multiply and cause either disseminated illness or chronic pulmonary disease.

Healthy young adults are unlikely to develop active MAC disease, and children with asthma don’t fit the classic risk profile. Elderly individuals with COPD have risk, but the key factor that makes MAC most likely is either impaired cellular immunity or preexisting lung architecture, which is why that population is the best answer.