A drug X reverses the actions produced by parasympathetic nervous system in the body but is not able to reverse the actions of exogenously administered acetylcholine in in-vitro experiments. The mechanism of drug X can be
**Core Concept**
The parasympathetic nervous system (PNS) is a branch of the autonomic nervous system (ANS) that promotes relaxation and restoration by stimulating the release of acetylcholine (ACh) from postganglionic neurons. Drug X appears to counteract the effects of PNS stimulation, but not the direct action of exogenously administered ACh.
**Why the Correct Answer is Right**
Drug X is likely an anticholinesterase inhibitor, such as physostigmine. It works by inhibiting the enzyme acetylcholinesterase (AChE), which normally breaks down ACh into choline and acetate. By preventing ACh degradation, anticholinesterase inhibitors increase the concentration and duration of action of ACh in the synaptic cleft. However, this effect is limited to endogenously produced ACh, as exogenously administered ACh is not subject to the same enzymatic degradation. As a result, anticholinesterase inhibitors can enhance the effects of PNS stimulation without reversing the direct action of exogenously administered ACh.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not explain how drug X could counteract the effects of PNS stimulation without reversing the direct action of exogenously administered ACh.
**Option B:** This option is incorrect because it does not accurately describe the mechanism by which drug X works.
**Option C:** This option is incorrect because it does not provide a plausible explanation for the observed effects of drug X.
**Clinical Pearl / High-Yield Fact**
Anticholinesterase inhibitors, such as neostigmine, are used to treat myasthenia gravis, a neuromuscular disorder characterized by weakness and fatigue. They work by increasing the concentration and duration of action of ACh at the neuromuscular junction.
**Correct Answer: C.**