A 79-year-old woman, previously independent and managing her own affairs, is brought to the emergency department by her daughter. She has had 2 days of drowsiness and confusion that varies through the day. Temperature is 38.4°C. She is inattentive, believes the year is 1985 and says she is at her old workplace. Urine dipstick shows leucocytes and nitrites. CRP is 118 mg/L (below 10), white cell count 16.1 × 10⁹/L (4.0–11.0) and blood glucose 5.6 mmol/L (3.0–6.0). She refuses admission and intravenous antibiotics, saying she is perfectly well, and cannot recall the risks you explained 5 minutes earlier. You conclude that she lacks capacity for this decision.

What is the most likely cause of her lack of capacity?