**Question:** Anticholinesterase that binds only with the anionic site is

A. Donepezil
B. Galantamine
C. Tacrine
D. Rivastigmine

**Correct Answer:** D. Rivastigmine

**Core Concept:**
Anticholinesterases are a class of medications used in the treatment of Alzheimer's disease and other cognitive disorders. They act as inhibitors of the enzyme acetylcholinesterase, which breaks down the neurotransmitter acetylcholine in the synaptic cleft. By inhibiting this enzyme, anticholinesterases increase the concentration of acetylcholine, thereby improving cognitive function.

**Why the Correct Answer is Right:**
Rivastigmine is a specific anticholinesterase that selectively binds to the anionic site of the acetylcholinesterase enzyme, leading to its inhibition. The anionic site is a negatively charged region on the enzyme where the anticholinesterase binds, preventing the enzyme from breaking down acetylcholine. In contrast, other anticholinesterases like donepezil, galantamine, and tacrine bind to both the anionic and orthosteric sites, leading to a higher risk of side effects and less potent inhibition of acetylcholinesterase.

**Why Each Wrong Option is Incorrect:**

A. Donepezil: This anticholinesterase binds to the orthosteric site, which is the binding site for acetylcholine. While this results in increased acetylcholine levels, donepezil also binds to the anionic site, leading to side effects.

B. Galantamine: Similar to donepezil, galantamine binds to both the anionic and orthosteric sites, resulting in increased acetylcholine levels but also increased side effects.

C. Tacrine: This anticholinesterase also binds to both the anionic and orthosteric sites, leading to increased acetylcholine levels but increased side effects.

**Clinical Pearl:**

Rivastigmine is a preferable choice among anticholinesterase medications as it selectively binds to the anionic site, leading to potent inhibition of acetylcholinesterase without significant side effects caused by binding to the orthosteric site. This results in improved efficacy and reduced side effects compared to donepezil, galantamine, and tacrine.