**Core Concept**
Autoimmune atrophic gastritis is a chronic condition characterized by inflammation and atrophy of the gastric mucosa, primarily affecting the antrum and body of the stomach. It is caused by an autoimmune response against parietal cells, leading to a decrease in gastric acid production and potentially resulting in vitamin B12 deficiency, iron deficiency anemia, and other complications.

**Why the Correct Answer is Right**
The condition is associated with the presence of autoantibodies against parietal cells and intrinsic factor, a protein necessary for vitamin B12 absorption. The autoimmune response leads to the destruction of parietal cells, which are responsible for producing gastric acid and intrinsic factor. This results in a decrease in gastric acid production and a malabsorption of vitamin B12, leading to megaloblastic anemia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because autoimmune atrophic gastritis primarily affects the body and antrum of the stomach, rather than the fundus.
**Option B:** This option is incorrect because the condition is characterized by a decrease in gastric acid production, not an increase.
**Option C:** This option is incorrect because autoimmune atrophic gastritis is not primarily associated with lymphocytic infiltration of the duodenum.

**Clinical Pearl / High-Yield Fact**
Autoimmune atrophic gastritis is often associated with other autoimmune conditions, such as pernicious anemia, type 1 diabetes, and Hashimoto's thyroiditis. It is essential to consider these associations when evaluating patients with autoimmune gastritis.

**Correct Answer: C.**