Correct Answer: A. N-acetylcysteine
Paracetamol (acetaminophen) overdose causes acute liver injury through formation of a toxic metabolite, N-acetyl-p-benzoquinone imine (NAPQI), which depletes hepatic glutathione stores and leads to hepatocellular necrosis. The patient's oliguria and deranged liver function tests indicate acute liver failure with potential progression to fulminant hepatic failure. N-acetylcysteine (NAC) is the gold-standard antidote for paracetamol poisoning in India and globally. NAC works by replenishing hepatic glutathione, directly conjugating NAPQI, and providing antioxidant protection. The efficacy of NAC is time-dependent—it is most effective when given within 8–10 hours of ingestion (the "golden window"), but remains beneficial even beyond this period, particularly in preventing progression to fulminant failure. NAC is administered intravenously in a loading dose (150 mg/kg over 1 hour), followed by maintenance infusions over 16–20 hours. In Indian clinical practice, NAC is the standard of care per AIIMS and other tertiary centre protocols. Early recognition and NAC administration can prevent the need for liver transplantation and reduce mortality in paracetamol-induced acute liver failure.
Why the other options are wrong
B. Ursodeoxycholic acid — Ursodeoxycholic acid is a bile acid used in chronic cholestasis and primary biliary cirrhosis to reduce hepatotoxicity, not in acute paracetamol poisoning. It has no role in neutralizing NAPQI or replenishing glutathione. This is an NBE trap—students may confuse it with a hepatoprotective agent, but it is ineffective in acute toxic liver injury and does not address the underlying mechanism of paracetamol toxicity. C. Dopamine — Dopamine is a vasopressor used in hypotension and cardiogenic shock, not in paracetamol poisoning. While oliguria may suggest renal hypoperfusion, dopamine does not treat the primary liver injury or prevent hepatocellular necrosis. This option may trap students who focus on oliguria as the main problem rather than recognizing that the liver failure is the primary pathology requiring specific antidotal therapy. D. Furosemide — Furosemide is a loop diuretic used for fluid overload and pulmonary edema, not for paracetamol poisoning. In fact, aggressive diuresis may worsen renal function in the setting of acute liver failure and oliguria. This is a distractor—students may incorrectly assume that diuretics will improve oliguria, but they do not address the underlying hepatotoxicity and may precipitate acute kidney injury.
High-Yield Facts
- N-acetylcysteine (NAC) is the antidote for paracetamol overdose; most effective within 8–10 hours of ingestion but beneficial even after 24 hours.
- NAPQI (N-acetyl-p-benzoquinone imine) is the toxic metabolite of paracetamol that depletes glutathione and causes hepatocellular necrosis.
- NAC dosing: Loading dose 150 mg/kg IV over 1 hour, then maintenance infusions; oral NAC (140 mg/kg loading, then 70 mg/kg every 4 hours × 17 doses) is alternative if IV unavailable.
- Paracetamol toxicity threshold: >150 mg/kg in a single ingestion or >4 g/day chronically; nomograms (Rumack-Matthew) guide NAC initiation based on serum levels and time post-ingestion.
- Fulminant hepatic failure from paracetamol requires ICU monitoring, coagulopathy correction, and consideration for liver transplant if INR >1.5 with encephalopathy.
Mnemonics
NAC for Paracetamol (NAPE) N-acetylcysteine for Acetaminophen (paracetamol) Poisoning Emergency. Replenishes glutathione, neutralizes NAPQI, prevents liver failure. Use this when you see paracetamol overdose + liver dysfunction. Golden Window Rule NAC is most effective within 8–10 hours of paracetamol ingestion (the 'golden window'). After 24 hours, efficacy drops but NAC still prevents progression to fulminant failure. Remember: 'Earlier is better, but never too late.'
NBE Trap
NBE pairs paracetamol overdose with oliguria to distract students toward renal/hemodynamic interventions (dopamine, furosemide) rather than the specific hepatic antidote. The oliguria is a consequence of liver failure, not the primary problem—recognizing this distinction is key.
Clinical Pearl
In Indian emergency departments, paracetamol overdose is common due to easy over-the-counter availability. Early NAC administration (even empirically, before serum levels return) in any suspected overdose with liver dysfunction can be life-saving and prevent the need for emergency liver transplantation—a resource-limited intervention in most Indian centres.
_Reference: Harrison Ch. 297 (Drug-Induced Liver Injury); KD Tripathi Ch. 8 (Analgesics & Antipyretics); Robbins Ch. 18 (Liver & Biliary Tract)_