Answer: Biopsy (endoscopy)
Explanation: Chronic gastritis with a loss of parietal cells due to atrophy (i.e degeneration of cells) is usually caused by bacterial infection (often H. Pylori).
Parietal cells are epithelial cells that line the gastric and secretes hydrochloric acid (HCl) and intrinsic factor in the stomach.
The best approach to diagnose the presence or loss of parietal cells is through biopsy. It is carried out by insertion of an endoscope through the mouth and down into the stomach to pick stomach cells samples. Then, the abundance and stability of the parietal cells is observed.