**Core Concept**
The patient's symptoms and decreased plasma cholinesterase level suggest an accumulation of acetylcholine, a neurotransmitter that plays a key role in the parasympathetic nervous system. This condition leads to overstimulation of muscarinic and nicotinic receptors, causing various systemic effects.

**Why the Correct Answer is Right**
The patient's symptoms, including pinpoint pupils (miosis), excessive salivation (hypersalivation), lacrimation, tremors, and red tears, are characteristic of organophosphate poisoning or acute cholinergic syndrome. The decreased plasma cholinesterase level indicates that the enzyme responsible for breaking down acetylcholine, acetylcholinesterase, is inhibited. This inhibition prevents the normal degradation of acetylcholine, leading to its accumulation and subsequent overstimulation of cholinergic receptors.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not directly relate to the patient's symptoms and decreased plasma cholinesterase level. While it may be a consideration in some cases, it is not the most probable diagnosis in this scenario.
* **Option B:** This option is incorrect as it is a symptom rather than a diagnosis. While the patient's symptoms are consistent with organophosphate poisoning, the option does not provide a clear diagnosis.
* **Option C:** This option is incorrect as it is not directly related to the patient's symptoms and decreased plasma cholinesterase level. While it may be a consideration in some cases, it is not the most probable diagnosis in this scenario.

**Clinical Pearl / High-Yield Fact**
In cases of suspected organophosphate poisoning, atropine is a commonly used antidote to counteract the effects of acetylcholine accumulation. Atropine works by competing with acetylcholine for binding to muscarinic receptors, thereby reducing the severity of symptoms.

**Correct Answer:** D. Organophosphate poisoning