**Core Concept**
Autoimmune gastritis is a condition where the immune system mistakenly attacks the parietal cells in the stomach, leading to a decrease in the production of intrinsic factor, a protein necessary for vitamin B12 absorption. This deficiency in vitamin B12 results in megaloblastic anemia.

**Why the Correct Answer is Right**
Autoimmune gastritis is a known cause of vitamin B12 deficiency anemia due to the destruction of parietal cells, which produce intrinsic factor. Intrinsic factor is essential for the absorption of vitamin B12 in the ileum. Without it, vitamin B12 cannot be absorbed, leading to a deficiency. This deficiency affects the production of red blood cells, resulting in megaloblastic anemia. The patient's normal reticulocyte count supports this diagnosis, as it indicates that the bone marrow is not responding to the anemia by increasing red blood cell production.

**Why Each Wrong Option is Incorrect**
**Option A:** Acute erosive gastritis is a condition characterized by inflammation and erosion of the stomach lining, which can lead to bleeding and pain. However, it is not typically associated with vitamin B12 deficiency anemia.

**Option C:** Helicobacter pylori gastritis is a bacterial infection that can cause inflammation and ulcers in the stomach. While it can lead to gastritis, it is not a direct cause of vitamin B12 deficiency anemia.

**Option D:** Menetrier disease is a rare condition characterized by overgrowth of the stomach lining, leading to protein loss and anemia. However, it is not typically associated with vitamin B12 deficiency anemia.

**Clinical Pearl / High-Yield Fact**
Intrinsic factor deficiency is a key factor in vitamin B12 deficiency anemia. Remember that autoimmune gastritis, pernicious anemia, and certain surgical procedures (e.g., gastrectomy) can lead to intrinsic factor deficiency and subsequent vitamin B12 deficiency anemia.

**✓ Correct Answer: B. Autoimmune gastritis. Autoimmune gastritis is a condition where the immune system mistakenly attacks the parietal cells in the stomach, leading to a decrease in the production of intrinsic factor, a protein necessary for vitamin B12 absorption.**