Why is the duration longer for MDR-TB regimens compared to drug-susceptible TB?

• A. Because resistant strains mutate rapidly
• B. Because resistant strains require prolonged treatment with multiple second-line drugs to achieve a cure and prevent relapse
• C. Because patients are less adherent
• D. Because they have stronger immune systems

Answer: (B)

Longer therapy for MDR-TB is needed because the bacteria causing this form are resistant to the most potent first-line drugs, so the regimen cannot kill the infection as quickly or as completely. Treating MDR-TB relies on a combination of second-line drugs that have different targets, but these drugs are generally less potent and have slower bactericidal and sterilizing activity. To achieve a durable cure and prevent relapse, the course must be long enough to eradicate both actively growing bacilli and persisting dormant bacteria. Using multiple drugs in combination also helps prevent the bacteria from developing additional resistance, which is more likely if treatment is too short. While patient adherence matters, the overarching reason for the longer duration is the need to overcome resistance with a less potent drug mix and to ensure complete, lasting eradication.