Final answer:
The underlying type of gastritis that causes pernicious anemia is chronic gastritis, which is caused by the gram-negative bacterium Helicobacter pylori.
Step-by-step explanation:
The underlying type of gastritis that causes pernicious anemia is chronic gastritis. Chronic gastritis is a condition characterized by inflammation of the lining of the stomach, usually caused by the gram-negative bacterium Helicobacter pylori. H. pylori can persist in the stomach by producing the enzyme urease, which modifies the acidity of the stomach.