Which of the following is NOT an acetylcholinesterase inhibitor?

• A. Donepezil
• B. Memantine
• C. Rivastigmine
• D. Galantamine

Answer: (B)

Memantine stands apart because it does not block acetylcholinesterase. Instead, it acts as an NMDA receptor antagonist, reducing excitotoxic glutamate signaling in the brain. This is a different mechanism from the cholinergic approach used by the other three drugs, which inhibit acetylcholinesterase to raise acetylcholine levels and improve synaptic transmission. Donepezil, Rivastigmine, and Galantamine are all acetylcholinesterase inhibitors (with Rivastigmine also inhibiting butyrylcholinesterase to some extent), making them cholinergic-enhancing treatments for Alzheimer's disease. Memantine is typically reserved for moderate to severe stages and is not an acetylcholinesterase inhibitor.