**Core Concept**
Sialorrhea, also known as hypersalivation or ptyalism, is an excessive production of saliva, which can be a side effect of various medications. This condition is often associated with impaired swallowing, drooling, and social embarrassment.

**Why the Correct Answer is Right**
Sialorrhea is a well-documented side effect of anticholinesterase inhibitors, such as neostigmine, pyridostigmine, and edrophonium. These medications inhibit the enzyme acetylcholinesterase, which breaks down the neurotransmitter acetylcholine. As a result, acetylcholine accumulates in the synaptic cleft, causing overstimulation of the parasympathetic nervous system, leading to excessive salivation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because antihistamines, such as diphenhydramine, are more likely to cause dry mouth (xerostomia) due to their anticholinergic effects.

**Option B:** This option is incorrect because beta blockers, such as propranolol, are not typically associated with sialorrhea.

**Option C:** This option is incorrect because antipsychotics, such as haloperidol, can cause dry mouth (xerostomia) due to their anticholinergic effects.

**Clinical Pearl / High-Yield Fact**
It is essential to note that medications with anticholinergic properties can cause dry mouth, while medications with anticholinesterase properties can cause excessive salivation.

**Correct Answer: C. antipsychotics**