**Core Concept**
Atropine poisoning involves an anticholinergic syndrome, characterized by tachycardia, mydriasis, dry mouth, and urinary retention. The primary goal of treatment is to restore cholinergic activity and counteract the effects of atropine.

**Why the Correct Answer is Right**
Physostigmine, a reversible cholinesterase inhibitor, is the antidote for atropine poisoning. It crosses the blood-brain barrier and increases the concentration of acetylcholine in the synaptic cleft, thereby counteracting the anticholinergic effects of atropine. Physostigmine also inhibits the breakdown of acetylcholine, allowing it to accumulate and exert its effects.

**Why Each Wrong Option is Incorrect**
**Option A:** Atropine itself is not an antidote; it is the toxin causing the poisoning.
**Option B:** Activated charcoal may be used to prevent further absorption of atropine, but it does not counteract its effects.
**Option C:** Flumazenil, a benzodiazepine antagonist, is not relevant to the treatment of atropine poisoning.

**Clinical Pearl / High-Yield Fact**
Physostigmine is a unique antidote because it not only counteracts the effects of atropine but also crosses the blood-brain barrier, making it effective in reversing central nervous system toxicity.

**Correct Answer:** D. Physostigmine