XDR-TB is defined as MDR plus resistance to which classes?

• A. MDR plus resistance to fluoroquinolones and at least one second-line injectable
• B. MDR only
• C. Resistance to isoniazid and ethambutol
• D. Resistance to all drugs

Answer: (A)

XDR-TB builds on MDR-TB by adding resistance to two major classes of second-line drugs: fluoroquinolones and at least one second-line injectable (such as amikacin, kanamycin, or capreomycin). Fluoroquinolones are among the most important oral options for TB treatment, and the injectables are critical alternatives when oral drugs fail. When MDR-TB is accompanied by resistance to both of these classes, treatment choices become extremely limited, which is why this exact combination defines XDR-TB. The other patterns don’t fit: MDR alone lacks the additional resistance in the two key second-line classes; resistance to only isoniazid and ethambutol involves first-line drugs and doesn’t specify the required second-line class resistance; resistance to all drugs would be a broader, different scenario not used in the formal definition of XDR-TB.