**Core Concept**
Organophosphorus poisoning is a type of cholinesterase inhibitor intoxication, leading to an accumulation of acetylcholine in the synaptic cleft. This results in overstimulation of muscarinic and nicotinic receptors, causing a range of systemic effects.

**Why the Correct Answer is Right**
The most specific test for organophosphorus poisoning is the measurement of red blood cell (RBC) acetylcholinesterase (AChE) activity. This enzyme is irreversibly inhibited by organophosphorus compounds, leading to a significant and prolonged decrease in its activity. The RBC AChE activity is a reliable indicator of organophosphorus poisoning, as it is less susceptible to fluctuations in plasma cholinesterase activity due to other causes.

**Why Each Wrong Option is Incorrect**
**Option A:** Plasma cholinesterase activity is not a specific test for organophosphorus poisoning, as it can be affected by various other factors, including genetic polymorphisms and liver disease.

**Option B:** The presence of mevinphos in urine is a useful indicator of organophosphorus poisoning, but it is not the most specific test.

**Option C:** Symptoms such as muscle cramps, diarrhea, and salivation are typical of organophosphorus poisoning, but they are not specific and can be seen in other conditions as well.

**Clinical Pearl / High-Yield Fact**
In cases of suspected organophosphorus poisoning, it is essential to measure RBC AChE activity to confirm the diagnosis, as the symptoms can be nonspecific and may overlap with other conditions.

**Correct Answer: D. Measurement of RBC acetylcholinesterase activity.**