This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Systemic Pathology, Subject name: Pathology
A 34-year-old man presents with 5-month history of weakness and fatigue. There is no history of drug or alcohol abuse. CBC shows megaloblastic anemia and a normal reticulocyte count. Fuher laboratory studies reveal vitamin B12 deficiency. Anemia in this patient is most likely caused by which of the following? 
Select and explain. 
A. Acute erosive gastritis, B. Autoimmune gastritis, C. Helicobacter pylori gastritis, D. Menetrier disease
B: Autoimmune gastritis
Answer: B: Autoimmune gastritis 
Explanation: Type A/ Autoimmune gastritis: -Site : Fundus & body of stomach - Damage : Parietal cells resulting in: Achlorhydria - |G-cell stimulation - |Gastrin levels -| Neuroendocrine tumor Intrinsic factor deficiency - Malabsorption of vitamin B12 - Megaloblastic anemia /Pernicious anemia