You are answering a USMLE-style medical question. Question: A 65-year-old man presents with a 6-month history of repeated falls, postural dizziness, progressive fatigue, generalized weakness and a 13.6 kg (30 lb) weight loss. He is a vegetarian. The patient’s family says that he had been high functioning but has had significant behavioral changes over the past year. The patient denies any smoking history, alcohol consumption, or illicit drug use. No significant family history. His vital signs include: blood pressure 90/50 mm Hg without postural changes, pulse 92/min, respiratory rate 16/min, temperature 37.0℃ (98.6℉). Physical examination reveals a poorly groomed, disheveled, thin man. He is irritable, paranoid, and delusional but denies any hallucinations. An unstable, wide-based ataxic gait is noted. Laboratory results are significant for the following: Hb 6.1 g/dL MCV 109 fL Platelets 90,0000/mm3 Total count 3,000/mm3 Reticulocyte count 0.8% A peripheral blood smear demonstrates hypersegmented neutrophils. Anti-intrinsic factor antibodies are negative. Which of the following is the most likely cause of this patient’s condition? Options: A) Hypothyroidism B) Folate deficiency C) Vitamin B12 deficiency D) Parvovirus infection Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.