A 67-year-old female with a history of autoimmune gastritis and strict vegetarian diet for 15 years presents with a 3-month history of progressive paranoid ideation, auditory hallucinations (hearing voices commenting on her actions), and disorganized thinking. Her daughter reports cognitive decline with memory impairment and difficulty managing finances. On examination, she has a smooth, beefy-red tongue, diminished vibratory sense and proprioception in both lower extremities, a positive Romberg sign, and bilateral extensor plantar responses (Babinski signs). Her gait is wide-based and unsteady. Complete blood count reveals hemoglobin of 9.8 g/dL with an MCV of 118 fL. Peripheral blood smear shows hypersegmented neutrophils and macro-ovalocytes. Serum vitamin B12 level is 68 pg/mL (normal 200-900 pg/mL), and methylmalonic acid is markedly elevated. Which of the following is the most likely diagnosis?