Regarding the standard 6-month regimen for drug-susceptible pulmonary tuberculosis, all of the following statements are correct EXCEPT:

Explanation

## Standard 6-Month TB Regimen Structure ### Intensive Phase (First 2 Months) The intensive phase uses a four-drug combination: **HRZE** (Isoniazid, Rifampicin, Pyrazinamide, Ethambutol). **Key Point:** Ethambutol's role in the intensive phase is **NOT** to prevent drug resistance — rather, it is added to cover potential isoniazid-resistant strains and to provide additional bactericidal activity. The primary drugs preventing resistance are isoniazid and rifampicin through their potent bactericidal action. ### Continuation Phase (Next 4 Months) The continuation phase uses **HR** (Isoniazid and Rifampicin alone) for 4 months. **High-Yield:** The standard regimen is: - **2HRZE / 4HR** = 6 months total - Intensive: 2 months of four drugs - Continuation: 4 months of two drugs ### Why Each Drug? | Drug | Phase | Purpose | |------|-------|----------| | Isoniazid | Both | Bactericidal, sterilizing activity | | Rifampicin | Both | Bactericidal, kills intracellular bacilli | | Pyrazinamide | Intensive only | Bactericidal in acidic pH, shortens duration | | Ethambutol | Intensive only | Prevents emergence of resistant strains; covers INH resistance | **Clinical Pearl:** Ethambutol is added to the intensive phase to **prevent the emergence of drug-resistant mutants** during the early bactericidal phase, not to prevent resistance per se. Its main role is as a companion drug to protect against pre-existing isoniazid resistance. **Warning:** The distractor conflates ethambutol's role. While it does contribute to preventing resistance, the statement "to prevent drug resistance" is imprecise — it is added because resistance may already exist (undetected INH resistance) or to prevent selection of resistant mutants. The more accurate statement is that it covers potential isoniazid-resistant strains. [cite:Harrison 21e Ch 158]