**Core Concept**
The question tests the student's knowledge of pharmacological management of Alzheimer's disease, specifically the current treatment landscape and the discontinuation of certain medications.

**Why the Correct Answer is Right**
Aricept (donepezil), Exelon (rivastigmine), and Reminyl (galantamine) are cholinesterase inhibitors that work by increasing the concentration of acetylcholine in the brain, thereby improving cognitive function in patients with Alzheimer's disease. However, the drug that is not used now in Alzheimer's disease is Tacrine, also a cholinesterase inhibitor. Tacrine was withdrawn from the market due to its hepatotoxicity and the availability of safer alternatives.

**Why Each Wrong Option is Incorrect**
**Option A:** Donepezil is still widely used and is considered a first-line treatment for Alzheimer's disease. It is a reversible cholinesterase inhibitor that improves cognitive function.
**Option B:** Rivastigmine is another cholinesterase inhibitor that is used in the treatment of Alzheimer's disease and Parkinson's disease dementia.
**Option C:** Galantamine is also a cholinesterase inhibitor that is used to treat Alzheimer's disease and is believed to have a favorable side effect profile compared to other medications in its class.

**Clinical Pearl / High-Yield Fact**
Cholinesterase inhibitors should be used with caution in patients with a history of gastrointestinal disorders, as they can exacerbate these conditions.

**Correct Answer: D. Tacrine.