A 68-year-old man with a 3-year history of progressive memory loss and difficulty recognizing family members is brought to the hospital by his son. Over the past 6 months, his cognitive decline has worsened gradually. On examination, he is alert and oriented to person only, with intact attention span. He can follow commands and has normal vital signs (BP 130/80, HR 78, RR 16, Temp 37°C). Blood investigations including CBC, electrolytes, glucose, and renal function are all normal. CT head shows cerebral atrophy. What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis **Key Point:** This patient demonstrates the classic insidious, progressive course of Alzheimer's disease dementia over 3 years, with gradual cognitive decline and preserved alertness. ### Distinguishing Features Present | Feature | This Case | Delirium | Dementia | |---------|-----------|---------|----------| | **Onset** | Gradual (3 years) | Acute (hours to days) | Insidious (months to years) | | **Course** | Progressive, steady decline | Fluctuating, waxing-waning | Slowly progressive | | **Alertness** | Alert and oriented | Altered (hyperactive/hypoactive) | Preserved until late stages | | **Attention** | Intact | Markedly impaired | Relatively preserved early | | **Vital signs** | Normal | Often abnormal | Usually normal | | **Labs** | All normal | Often abnormal | Usually normal | | **Reversibility** | Irreversible | Often reversible | Irreversible | **High-Yield:** The 3-year progressive history with normal vitals, normal labs, and preserved alertness/attention rules out delirium entirely. The gradual memory loss and disorientation to place/time (but not person) is typical of Alzheimer's disease. **Clinical Pearl:** Dementia patients maintain clear consciousness and attention until advanced stages; delirium patients show acute fluctuation in both. This patient's steady 3-year decline with intact attention span is pathognomonic for dementia. ### Why Alzheimer's Disease? 1. Insidious onset over years (not acute) 2. Progressive memory loss as primary feature 3. Preserved alertness and vital signs 4. Normal metabolic workup (rules out reversible causes) 5. CT atrophy consistent with neurodegenerative disease **Mnemonic: DEMENTIA vs DELIRIUM** — **D**ementia = **D**ays to years (slow), **D**elirium = **D**ays (acute); **D**ementia = **D**ark (preserved consciousness), **D**elirium = **D**isturbed (altered consciousness).