**Core Concept**
Organophosphorous poisoning inhibits acetylcholinesterase, an enzyme crucial for terminating the action of acetylcholine at cholinergic synapses. Monitoring the activity of this enzyme is essential for diagnosing and managing organophosphorous poisoning.

**Why the Correct Answer is Right**
Plasma cholinesterase level is the most specific test for organophosphorous poisoning because it directly reflects the inhibition of acetylcholinesterase. This enzyme is responsible for breaking down acetylcholine into choline and acetic acid. Inhibition of acetylcholinesterase leads to an accumulation of acetylcholine, causing excessive stimulation of muscarinic and nicotinic receptors. Plasma cholinesterase level is a sensitive indicator of this enzyme's activity, making it a valuable diagnostic tool.

**Why Each Wrong Option is Incorrect**
**Option A:** RBC cholinesterase level is less specific than plasma cholinesterase level because it can be affected by various factors, including hemolysis and enzyme redistribution.

**Option C:** RBC uroporphyrin level is unrelated to organophosphorous poisoning and is instead associated with porphyria, a group of disorders affecting heme biosynthesis.

**Option D:** Measurement of serum level of organophosphorous is not a specific test for poisoning and can be influenced by various factors, including exposure duration and route of exposure.

**Clinical Pearl / High-Yield Fact**
In cases of suspected organophosphorous poisoning, it's essential to monitor plasma cholinesterase levels regularly to assess the effectiveness of treatment and guide clinical decision-making.

**✓ Correct Answer: B. Plasma cholinestrase level**