A patient with pernicious anemia asks why injections are lifelong. Best explanation?

Pernicious anemia results from an autoimmune destruction of the gastric parietal cells that normally produce intrinsic factor. Intrinsic factor is a glycoprotein essential for the absorption of vitamin B12 (cobalamin) in the terminal ileum. Without it, orally ingested B12 — whether from food or oral supplements — cannot be taken up from the gastrointestinal tract in meaningful amounts. Intramuscular or subcutaneous injections bypass the GI tract entirely, delivering B12 directly into the bloodstream, where it can reach tissues and the liver for storage. Because the underlying autoimmune destruction of parietal cells is permanent and not reversible with current treatments, the patient will never recover the ability to absorb dietary B12. Lifelong injections are therefore necessary to maintain adequate serum and tissue levels. The option stating that pills are less effective is imprecise and misleading — oral B12 in very high doses can partially compensate through passive diffusion in some patients, but this does not apply to the classic pernicious anemia explanation. The claim that injections prevent GI upset is not the primary rationale. The statement that the liver cannot store B12 is factually incorrect; the liver stores large amounts of B12. The central mechanism is always the absence of intrinsic factor preventing intestinal absorption, making that the best and most accurate explanation for the patient.