A mainly home-bound retired building contractor aged 68 years with well-controlled hypertension and a 2-year history of mild Alzheimer's disease (AD) was brought to the emergency department (ED) by his wife because of the relatively sudden onset of noticeably increasing cognitive impairment during the past 4 days, which was affecting his daily living activities. He was not under any unusual stress but had fallen the previous day due to "dizziness" (described as a feeling of lightheadedness and unsteadiness) and blurred vision. The patient has been treated with hydrochlorothiazide 25 mg every other day for hypertension for the past 6 years. An ophthalmologic examination within the past 2 months was normal. His wife reported to the ED physician that her husband began treatment with an anti-amyloid beta (Aβ) monoclonal antibody (mAb) 3 months ago for his mild AD. A baseline MRI prior to initiating treatment with the anti-Aβ mAb was normal. Examination in the ED revealed confusion, mild dizziness when standing, and deep tendon reflexes 0 to 1+ bilaterally but no nystagmus or other abnormal neurologic signs. He denied nausea/vomiting or headache. His temperature was 97.1 °F and his BP ranged from 132/82 to 158/94 mm Hg when taken multiple times.