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In pernicious anemia, the fundamental issue is the body's inability to absorb vitamin B12 due to a lack of intrinsic factor, which is a protein produced by the stomach that is essential for the absorption of this vitamin in the intestines. Without adequate vitamin B12, the body cannot produce enough healthy red blood cells, leading to large, improperly formed red blood cells characteristic of megaloblastic anemia.

This form of anemia is specifically linked to vitamin B12 deficiency and its symptoms such as fatigue, weakness, and pallor, as well as potential neurological issues like peripheral neuropathy. The inability to absorb vitamin B12 is therefore a critical and direct finding in patients with pernicious anemia, differentiating it from other types of anemia that may not have this absorption issue.

While the other options may reference different hematological conditions or findings, they do not specifically address the core mechanism of pernicious anemia, which revolves around vitamin B12 absorption. Thus, the presence of impaired vitamin B12 absorption stands out as a defining characteristic of this condition.