A 48-year-old woman presents with progressive abdominal distension and pelvic pain. Pelvic ultrasound reveals bilateral solid ovarian masses with ascites. The structure marked **A** in the diagram shows massive bilateral ovarian enlargement while preserving the bean-shaped ovarian contour. Histopathology of the ovarian biopsy reveals signet-ring cells with mucin-positive staining diffusely infiltrating the ovarian stroma. Upper endoscopy identifies a gastric antral ulcerated lesion with signet-ring cell adenocarcinoma. Which of the following best describes the pathological diagnosis and its primary origin?