How is multidrug-resistant TB defined?

• A. Resistance to rifampin only
• B. Resistance to pyrazinamide and ethambutol
• C. Resistance to at least isoniazid and rifampin
• D. Resistance to all first-line drugs

Answer: (C)

The main idea is that multidrug-resistant TB is defined by resistance to the two most potent first-line drugs: isoniazid and rifampin. If a TB strain cannot be killed by both of these drugs, standard short-course therapy is unlikely to work, so a different, longer, and more specialized treatment plan using second-line drugs is needed. Isoniazid and rifampin are the backbone of first-line treatment, so losing protection from both undermines the regimen’s effectiveness.

Resistance to rifampin alone would be considered mono-rifampin resistance, not MDR, until isoniazid resistance is also present. Resistance to pyrazinamide and ethambutol does not meet the criterion for MDR-TB because it does not involve both isoniazid and rifampin. Resistance to all first-line drugs is a more severe category (pan-drug resistance) and goes beyond the MDR definition.