A 27-year-old gentleman presented with 6 months of progressive behavior changes characterized by wandering tendencies, persecutory delusions, apathy, mutism, irritability, crying spells, disturbed sleep, and a decline in bio-socio-occupational functioning. He was diagnosed with psychosis and started on risperidone 4 mg/ day on an outpatient basis, but was brought back with symptom worsening in a month. He developed aggression, urinary incontinence, gait ataxia, and memory disturbances and was admitted. He was disoriented to time, place, and person, and had ataxic gait with a positive Romberg’s test. Vibration and proprioception sense were normal. Ophthalmoscopy revealed bilateral blurred optic disc margins. His Mini-Mental State Exam (MMSE) score was 8. A clinical diagnosis of dementia with delirium was made. Investigations revealed a hemoglobin of 11.4 gm/dl (MCV-107.4 fl); peripheral blood smear showed hypersegmented neutrophils. White blood cell count, platelets, electrolytes, and hepatic, renal, and thyroid function tests were normal. CT scan of the brain revealed diffuse cerebral atrophy with exvacuo-dilation of ventricles. Serum vitamin-B12 levels were 56.4 pg/ml (normal levels: 243–894 pg/ml). Gastric and intestinal mucosal study was normal. Dietary evaluation disclosed a very low intake of meat and other animal products.