A 42-year-old Indian man with a history of chronic diarrhea and steatorrhea for 2 years presents with progressive weakness and cognitive slowing. On examination, he has glossitis, mild ataxia, and loss of proprioception in the lower limbs. His hemoglobin is 8.5 g/dL, MCV is 108 fL, and WBC is 2800/μL with hypersegmented neutrophils. Serum folate is 2.1 ng/mL (normal: 5.4–16 ng/mL) and serum B12 is 95 pg/mL (normal: 200–900 pg/mL). Methylmalonic acid (MMA) is elevated at 3.8 μmol/L (normal: <0.4 μmol/L). Which of the following is the most likely underlying etiology?