参考答案：D

解析：

Alzheimer's disease is the most common cause of dementia in the elderly. Approximately 10% of people over age 65 have Alzheimer's disease; about 50% of people over age 85 have the disease. Delirium, or acute confusion, is caused by an underlying disease and isn't itself a cause of dementia. Depression is common in the elderly but, in many cases, manifests itself in apathy, self-deprecation, or inertia--not dementia. Excessive drug use, commonly stemming from the client seeing multiple physicians who are unaware of drugs that other physicians have prescribed, can cause dementia. Although it's a problem among the elderly, it isn't as common as Alzheimer's disease.