Q. A 45-year-old woman with a history of chronic alcoholism presents to the clinic complaining of weakness and fatigue. She also mentions that she has been feeling short of breath even with minor activities. On physical examination, she appears pale and has mildly icteric sclerae. Laboratory findings reveal macrocytic anemia, hypersegmented neutrophils, and elevated homocysteine levels, but normal methylmalonic acid levels. Given her alcoholism and poor nutritional status, her physician suspects a deficiency in a particular vitamin as the cause of her symptoms. What is the main pathophysiological process responsible for the megaloblastic anemia seen in this patient?