A 35-year-old man with newly diagnosed pulmonary tuberculosis is started on standard first-line therapy. Regarding adverse effects of antitubercular drugs, all of the following statements are true EXCEPT:

Explanation

## Adverse Effects of First-Line Antitubercular Drugs ### Adverse Effect Profile | Drug | Major Adverse Effect | Mechanism | Reversibility | |------|---------------------|-----------|----------------| | **Isoniazid** | Peripheral neuropathy | Vitamin B6 depletion | **Reversible** with B6 supplementation | | **Rifampicin** | Enzyme induction | CYP450 induction | Reversible upon discontinuation | | **Pyrazinamide** | Hyperuricemia | ↓ Renal uric acid excretion | Reversible | | **Ethambutol** | Optic neuritis (red-green color blindness) | Direct toxic effect on optic nerve | **Reversible if caught early** | ### Key Point: **Ethambutol-induced optic neuritis is REVERSIBLE if the drug is discontinued promptly.** This is a critical exam point. While prolonged exposure can lead to permanent vision loss, early recognition and discontinuation typically result in recovery of visual acuity and color vision. ### High-Yield: **Monitoring requirements for first-line drugs:** - **Isoniazid:** Baseline and periodic LFTs; give pyridoxine 10 mg/day prophylactically - **Rifampicin:** Baseline and periodic LFTs; counsel on drug interactions - **Pyrazinamide:** Baseline and periodic LFTs and uric acid; monitor for gout symptoms - **Ethambutol:** **Baseline visual acuity and color vision testing; monthly monitoring during treatment** ### Clinical Pearl: Ethambutol-induced optic neuritis typically presents with: - Decreased visual acuity - Red-green color blindness (initially) - Scotoma (central or peripheral) - Symptoms are dose-dependent and duration-dependent **If caught within 2–4 weeks of symptom onset and the drug is discontinued, vision usually recovers completely.** Delayed recognition leads to irreversible damage. ### Warning: **Do NOT confuse reversibility timing:** Ethambutol toxicity is reversible if caught early, but becomes irreversible with prolonged exposure. The question stem says "irreversible even after drug discontinuation" — this is FALSE if caught early, making this the correct answer for an "all true EXCEPT" question.