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

Explanation

## Definition of XDR-TB **Key Point:** XDR-TB (Extensively drug-resistant TB) is defined as **MDR-TB PLUS resistance to at least one fluoroquinolone (FQ) AND at least one second-line injectable (SLI) agent**. ### Core Components 1. **MDR-TB baseline** = INH + RIF resistance 2. **+ Fluoroquinolone resistance** (levofloxacin or moxifloxacin) 3. **+ Second-line injectable resistance** (amikacin, kanamycin, or capreomycin) ### TB Resistance Classification Pyramid ```mermaid graph TD A[Drug-susceptible TB]:::outcome B[INH or RIF resistance only]:::outcome C[MDR-TB<br/>INH + RIF]:::outcome D[Pre-XDR-TB<br/>MDR + FQ OR SLI]:::outcome E[XDR-TB<br/>MDR + FQ + SLI]:::urgent F[Beyond-XDR-TB<br/>XDR + bedaquiline/linezolid]:::urgent A --> B B --> C C --> D C --> E E --> F ``` ### Key Distinctions | Classification | INH + RIF | FQ | SLI | Treatment | |---|---|---|---|---| | **MDR-TB** | ✓ | ✗ | ✗ | 18–20 months (2nd-line) | | **Pre-XDR-TB** | ✓ | ✓ OR ✗ | ✗ OR ✓ | 18–20 months | | **XDR-TB** | ✓ | ✓ | ✓ | 20–24 months (3rd-line) | **High-Yield:** XDR-TB REQUIRES resistance to BOTH a fluoroquinolone AND an injectable — not just one. This distinction is critical for exam purposes. **Mnemonic:** **FQ + SLI = XDR** — Fluoroquinolone + Second-Line Injectable = eXtensively Drug-Resistant. **Clinical Pearl:** XDR-TB has significantly worse treatment outcomes (cure rates ~50–60%) and requires newer agents like bedaquiline, linezolid, and delamanid.