Dementia is most likely because:
1. Dr Zhengs cognitive decline had a gradual onset and a progressive course (delirium has an acute onset and is worse in the evening, while depression has a variable onset and symptoms can be worse in the morning).
2. Observation and assessment showed Dr Zheng has normal psychomotor behaviour, normal perception and normal attention. Delirium is characterised by impaired attention and perception and variable psychomotor behaviour (either very active/agitated or unusually quiet and withdrawn). People with depression can be agitated with variable attention but normal perception.
3. Dr Zheng has immediate and recent memory impairment, as indicated by his recent history of missing appointments. This is a required DSM-IV criteria for a diagnosis of dementia.