**Core Concept**
N-acetylcysteine (NAC) is the antidote of choice for paracetamol (acetaminophen) poisoning, acting as a glutathione precursor to replenish depleted liver stores and prevent liver damage.

**Why the Correct Answer is Right**
Paracetamol is metabolized by cytochrome P450 enzyme CYP2E1 to a toxic metabolite, N-acetyl-p-benzoquinone imine (NAPQI). NAPQI depletes glutathione stores in the liver, leading to oxidative stress and cell death. NAC replenishes glutathione levels, allowing the liver to neutralize NAPQI and prevent liver damage. The typical treatment involves administering NAC within 8-10 hours of paracetamol ingestion to be effective.

**Why Each Wrong Option is Incorrect**
**Option A:** Activated charcoal is not effective in treating paracetamol poisoning as it does not bind the toxic metabolite NAPQI.
**Option B:** Penicillin is an antibiotic and has no role in treating paracetamol poisoning.
**Option C:** Vitamin K is used to reverse warfarin anticoagulation and has no role in treating paracetamol poisoning.

**Clinical Pearl / High-Yield Fact**
NAC is most effective when administered within 8-10 hours of paracetamol ingestion, but can still be beneficial up to 24 hours if the patient presents early and meets specific criteria.

**Correct Answer: A. N-acetylcysteine**