**Core Concept**
Rivastigmine is a cholinesterase inhibitor used in the management of cognitive symptoms in patients with Alzheimer's disease and Parkinson's disease dementia. It works by increasing the concentration of acetylcholine in the brain, thereby enhancing cholinergic transmission.

**Why the Correct Answer is Right**
Rivastigmine inhibits both acetylcholinesterase and butyrylcholinesterase, leading to an increase in acetylcholine levels in the central nervous system. This results in improved cognitive function, including memory and attention, in patients with Alzheimer's disease and Parkinson's disease dementia. The drug's mechanism of action is similar to that of other cholinesterase inhibitors, such as donepezil and galantamine.

**Why Each Wrong Option is Incorrect**
**Option A:** Rivastigmine is not primarily used in the treatment of major depressive disorder. While depression can coexist with dementia, rivastigmine's primary indication is for cognitive symptoms.
**Option B:** Rivastigmine is not a selective serotonin reuptake inhibitor (SSRI). SSRIs, such as fluoxetine and sertraline, are used to treat depression and anxiety disorders.
**Option C:** Rivastigmine is not an atypical antipsychotic. Atypical antipsychotics, such as risperidone and quetiapine, are used to treat schizophrenia and bipolar disorder.

**Clinical Pearl / High-Yield Fact**
Rivastigmine can cause cholinergic side effects, including nausea, vomiting, and diarrhea, due to the increased levels of acetylcholine in the brain. These side effects can be managed with dose titration and concomitant administration of anticholinergic medications.

**Correct Answer: C. Parkinson's disease dementia.**