A previously healthy Japanese 36-year-old man presented to the gastroenterology department with a 2-day history of acute-onset severe epigastric pain after eating flounder sashimi. His vital signs were normal. Emergency endoscopy showed a submucosal tumor-like lesion with mild erosions in the upper body of the stomach (Fig. 1, Panel A). Further observation revealed a whitish linear worm penetrating the mucosa at the edge of the lesion (Fig. 1, Panels B and C). What is the diagnosis?