An 80-year-old man with baseline mild cognitive impairment is hospitalized for pneumonia. On hospital day 2, his family notes acute confusion, agitation alternating with somnolence, and disorganized speech that fluctuates over the day. His baseline cognition was intact 48 hours ago. The most likely diagnosis is:
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A
Rapidly progressing Alzheimer disease
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B
Delirium
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C
New-onset psychosis
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D
Lewy body dementia