Pathophysiology
Clinical meaning
Drug-resistant TB emerges through selection pressure from inadequate treatment regimens, poor adherence, or transmission of resistant strains. MDR-TB is resistant to at least isoniazid and rifampin. XDR-TB (extensively drug-resistant) is MDR-TB plus resistance to a fluoroquinolone and at least one injectable agent. Treatment requires prolonged courses (9-20+ months) with more toxic second-line drugs. The BPaL regimen (bedaquiline, pretomanid, linezolid) represents a breakthrough for treatment-resistant cases.