A 32-year-old man from Delhi presents with a 3-month history of productive cough, fever, and night sweats. Chest X-ray shows cavitary lesions in the right upper lobe. Sputum smear microscopy is positive for acid-fast bacilli. He is started on standard four-drug antitubercular therapy (HRZE). After 6 weeks of treatment, he develops acute onset jaundice, dark urine, and right upper quadrant tenderness. Liver function tests show ALT 320 U/L, AST 280 U/L, total bilirubin 4.2 mg/dL, and INR 1.8. Which drug is most likely responsible for this hepatotoxicity and should be temporarily withheld?

Explanation

## Clinical Presentation Analysis The patient presents with classic features of drug-induced hepatotoxicity during antitubercular therapy: - Acute onset jaundice and dark urine - Right upper quadrant tenderness - Markedly elevated transaminases (ALT > AST) - Elevated bilirubin and INR (indicating synthetic dysfunction) ## Hepatotoxicity Profile of First-Line Drugs | Drug | Hepatotoxicity Incidence | Mechanism | Severity | Reversibility | |------|--------------------------|-----------|----------|---------------| | **Pyrazinamide** | 1–3% | Direct hepatocellular injury; metabolite accumulation | **Severe** | Reversible if caught early | | Isoniazid | 0.5–1% | Metabolite-mediated; slow acetylators at risk | Mild-moderate | Reversible | | Rifampicin | 0.5–1% | Induction of hepatic metabolism; rare direct injury | Mild-moderate | Reversible | | Ethambutol | <0.1% | Minimal hepatotoxicity | Minimal | N/A | ## Key Point: **Pyrazinamide is the most hepatotoxic of the first-line antitubercular drugs.** It causes dose-dependent hepatocellular injury, especially in the first 2–8 weeks of therapy. ## Clinical Pearl: **High-Yield:** Pyrazinamide-induced hepatotoxicity presents with: - Acute hepatitis picture (ALT > AST) - Jaundice and dark urine - Risk factors: advanced age, pre-existing liver disease, slow acetylator phenotype, high body weight - Management: Withhold PZA; restart at lower dose or switch to alternative regimen (HRZE → HRE + fluoroquinolone) ## Mnemonic: **PZA = Potent Hepatotoxin** — Among first-line drugs, pyrazinamide carries the highest risk of severe hepatotoxicity. ## Management Algorithm ```mermaid flowchart TD A[Patient on HRZE develops jaundice + ↑ LFTs]:::outcome A --> B{Identify offending drug}:::decision B -->|ALT > AST, acute onset| C[Pyrazinamide most likely]:::action B -->|Cholestasis pattern| D[Rifampicin less likely]:::action C --> E[Withhold PZA immediately]:::action E --> F[Restart after LFTs normalize]:::action F --> G[Consider lower dose or alternative]:::action ``` ## Why Pyrazinamide? Pyrazinamide is metabolized to **pyrazinoic acid**, which accumulates in hepatocytes and causes direct cytotoxic injury. The incidence of clinically significant hepatotoxicity (1–3%) is 2–5× higher than isoniazid or rifampicin. Early detection and withdrawal usually result in complete resolution within 2–4 weeks. [cite:KD Tripathi 8e Ch 47]