Which statement defines MDR-TB?

• A. MDR-TB is resistance to at least isoniazid and rifampin.
• B. MDR-TB is resistance to rifampin and ethambutol.
• C. MDR-TB is resistance to isoniazid and ethambutol.
• D. MDR-TB is any resistance to first-line TB drugs.

Answer: (A)

MDR-TB is defined by resistance to the two most potent first-line TB drugs: isoniazid and rifampin. Losing both of these drugs makes the standard short-course regimen ineffective, which is why this specific pair is used as the defining criterion. Additional drug resistances may be present, but the minimum requirement for MDR-TB is that isoniazid and rifampin are both resisted.

If TB is resistant to rifampin and ethambutol, or to isoniazid and ethambutol, it doesn’t meet the standard definition because the other key drug isn’t involved in the resistance pattern. Likewise, resistance to any first-line drug alone isn’t enough to call it MDR-TB, since MDR specifically requires resistance to both isoniazid and rifampin.