**Core Concept**
Organophosphorus poisoning is a well-known medical emergency caused by the inhibition of acetylcholinesterase enzyme, leading to an accumulation of acetylcholine in the nervous system. This results in overstimulation of muscarinic and nicotinic receptors, manifesting as various systemic symptoms.

**Why the Correct Answer is Right**
Organophosphorus compounds, commonly found in pesticides and nerve agents, irreversibly bind to acetylcholinesterase, the enzyme responsible for breaking down acetylcholine. This leads to an accumulation of acetylcholine in the synaptic cleft, causing overstimulation of muscarinic receptors (responsible for parasympathetic effects) and nicotinic receptors (involved in neuromuscular transmission). The resulting symptoms include excessive salivation, lacrimation, miosis, fasciculations, tachycardia, and hypotension, as observed in the child. The inhibition of acetylcholinesterase also affects the neuromuscular junction, causing muscle fasciculations and weakness.

**Why Each Wrong Option is Incorrect**
**Option A:** Opium primarily affects the central nervous system by interacting with opioid receptors, leading to sedation, analgesia, and respiratory depression. It does not cause the peripheral symptoms seen in the child.
**Option C:** Dhatura (Datura stramonium) contains anticholinergic alkaloids that inhibit muscarinic receptors, resulting in symptoms such as dry mouth, blurred vision, and mydriasis (dilated pupils). This is opposite to the effects seen in organophosphorus poisoning.
**Option D:** Organochlorine pesticides, such as DDT, primarily affect the nervous system by disrupting sodium channels and causing seizures, tremors, and other neurological symptoms. They do not cause the characteristic muscarinic symptoms of organophosphorus poisoning.

**Clinical Pearl / High-Yield Fact**
When suspecting organophosphorus poisoning, it's essential to remember that atropine, an anticholinergic agent, is used as an antidote to counteract the muscarinic effects. However, atropine does not reverse the nicotinic effects, and patients may still experience muscle weakness and fasciculations.

✓ Correct Answer: B. Organophosphorus