An obese patient undergoes a gastric bypass procedure in order to lose weight but never returns for follow-up or continuing care. Three years later, he presents to an emergency room with fatigue, a glossy tongue, and a macrocytic and hyperchromic anemia. Which one of the following is deficient or malfunctioning in this patient, leading to this anemia?

The patient has developed a vitamin B12 deficiency, causing a megaloblastic anemia. Intrinsic factor is produced by gastric parietal cells (bypassed by this operation), which are required for vitamin B12 absorption by the terminal ileum. Without intrinsic factor, vitamin B12 cannot be absorbed. Because vitamin B12 is stored and efficiently recycled by the body, it can take a year or more to show deficiency. Vitamin B12 is routinely supplemented after this operation for life. Iron deficiency and lead poisoning would give a microcytic, hypochromic anemia. Valine is an essential amino acid, but if the patient is eating appropriately, the levels of valine should be sufficient for protein synthesis. Gastrin deficiency would decrease acid production in the stomach, but not lead to an anemia (a reduction in red blood cell count).