Final answer:
The most likely cause of a Vitamin B12 deficiency in older adults is Atrophic gastritis. This condition diminishes the stomach's ability to produce gastric acid and intrinsic factor, both essential for the proper absorption of vitamin B12.
Step-by-step explanation:
A deficiency of Vitamin B12 in older adults is most likely due to Atrophic gastritis. Atrophic gastritis is a condition that involves chronic inflammation of the stomach mucosa, leading to the loss of gastric glandular cells and resulting in diminished gastric acid secretion. As gastric acid is crucial for the release of vitamin B12 from food, a reduction in gastric acid can lead to a deficiency in this vitamin. Additionally, atrophic gastritis impairs the production of intrinsic factor, which is necessary for vitamin B12 absorption in the small intestine. The connection between atrophic gastritis and vitamin B12 deficiency is highlighted by the fact that individuals who undergo a total gastrectomy require vitamin B12 injections due to loss of intrinsic factor production from the stomach. In contrast, while conditions like diverticulitis, fat malabsorption, and gallbladder disease can also impact nutrition absorption, they are not directly tied to a B12 deficiency as strongly as atrophic gastritis is.