**Core Concept**
N-acetylcysteine (NAC) is the primary antidote for paracetamol (acetaminophen) overdose, acting as a glutathione precursor to counteract the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI).

**Why the Correct Answer is Right**
NAC replenishes glutathione stores, which are depleted in the setting of paracetamol overdose, thereby preventing the formation of NAPQI. NAPQI is a highly reactive metabolite responsible for hepatocellular damage. The administration of NAC within 8-10 hours of ingestion is crucial to prevent liver failure.

**Why Each Wrong Option is Incorrect**
**Option A:** Activated charcoal is not effective in paracetamol overdose as it cannot bind NAPQI, the toxic metabolite responsible for liver damage.
**Option B:** Penicillin is an antibiotic with no role in the management of paracetamol overdose.
**Option C:** Vitamin K is used in the management of warfarin overdose, not paracetamol overdose.

**Clinical Pearl / High-Yield Fact**
In patients with paracetamol overdose, early administration of NAC within 8 hours of ingestion significantly reduces the risk of liver failure.

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