A 32-year-old female accountant presents with a 6-month history of epigastric burning, postprandial fullness after normal-sized meals, and early satiety, worse during periods of high work stress. She denies weight loss, dysphagia, hematemesis, or melena. H. pylori stool antigen is negative. Upper GI endoscopy is performed. The structure marked **A** in the diagram shows endoscopically normal mucosa throughout the esophagus, stomach, and duodenum, with normal rugal folds, pristine duodenal villi, and no ulcers or erosions. Random gastric biopsies confirm normal histology with no H. pylori or intestinal metaplasia. Based on these findings and the clinical presentation meeting ROME IV criteria, what is the most likely diagnosis?